They have not fixed it, but it got reduced a little
The question about Linear fighters, Quadratic Wizards is a question about the balance between martials and casters. To summarize, while a martial gains a +X upgrade every level (+1, +2, +3, etc), a caster gains a +X^Y upgrade every level (+1 and charm, +2 and fly, +3 and teleportation, etc). To better understand the topic, the fine answers to this other question should be viewed first.
Spells are...Spells, and it's difficult to balance the creativity used to write new spells to "fighting better...somehow?". The versatility added with feats will never compete with spells until they decide that feats should be more versatile and do more than one or two things, or require multiple other feats. Spells are usually standalone and will do very well what they are proposed to do, with their effectiveness usually only being limited by the caster's creativity. It's not uncommon for players to find new uses for spells that were beyond the developer's imagination when he wrote it.
However, by reading all the classes so far in the playtest, watching some actual play podcasts and developer interviews, and frequently browsing the playtest forums, only one seems to be lacking both in the power potential and versatility, the alchemist. Fighters, paladins, rangers, rogues, and monks will do many more things related to being competent in combat, but little else outside of combat, unlike spellcasters in general.
The way things scale now in 2e brought numbers much closer together, as weapon damage will also scale up as you level up and spells will only scale in damage if you actually use a higher spell slot. But you won't see a fighter casting the equivalent to a fireball (which has limited daily uses) with their actions, even if they could do similar damage with a well-placed whirlwind attack, they get it at least 9 levels too late (14th vs 5th). The design idea still seems to be comparing unlimited uses versus limited uses to balance abilities between martials and casters. Martials get unlimited uses per day, while casters get limited uses with their spell slots, and as such, spells are stronger.
While at 5th level a wizard is already able to fly and rain 3d6 damage cantrips at foes for a whole combat, the 5th level fighter has abilities to hit two enemies at once or to gain +2 to AC until the start of their next turn.
But you said it got reduced? Yes, I did say it and it did, they merged many abilities from several feats directly into the classes, and those are all choices you can take (class feats). Feats are now more focused on skill usage and out of combat utility. Martials get many more options as they gain more combat-related choices while leveling up, but those are nearly always limited to combat utility.
The reliance on other feats is pretty much gone, you will get feat that makes previous feats better, but not completely different feats that require another previous feat that really has no relation to what the new feat does (like weapon focus and weapon specialization). For instance, a fighter that took Shield Block, (a reaction to raise your shield to absorb damage) can, later on, get Quick Shield Block, a feat that grants a free reaction that can be used only for shield blocking. Or a Double Shot feat can be upgraded to a Triple Shot, which works similarly but instead of using two actions, will use all three actions at a reduced penalty.
Think all the old combat feats baked directly into the classes now.
As for casters, while they do get way more utility in spells, the numbers of single-targeted damaging abilities are not that far from what martials can do. The DCs of spells got relatively reduced and will scale more slowly as they level up, on pair with how your saves will scale up too.
Remember when I said that spells won't scale up unless you use a higher spell slot? That is a problem often pointed out in these discussions about Quadratic Wizards, and let me try to explain.
In 3.PF, when a wizard levels up from 5th level to 6th, not only they gained an extra use of fireball, but their fireballs also got stronger. And this was even worse when they would gain new spell slots, as they could prepare their low level spells into higher level slots for the same effects (instead of 4 fireballs, you now can cast 7), but also had the benefits of using metamagic feats to increase the spell effect even more.
This passive scaling is gone in 2e for every spell but cantrips (just like happened in d&d 5e) and powers (which are similar to supernatural and spell-like abilities). To increase the damage output of your fireball, you gotta prepare the spell using a higher level slot. But this isn't exclusive to damaging spells, mage armor will start at +2 AC and get +1 AC for every slot level above first, healing spells will increase their number of dice by using higher level slots, and utility spells will allow you to affect more targets, or increase their duration (from 1 minute to 1 hour), or cause a stronger effect that has a broader utility (from humanoid creature to any creature), or even changing the possible targets (from Personal to Touch).
Casters still grow exponentially more useful and versatile for every extra level of spells they get, but their older spells won't automatically scale up when they learn new spells. If used on the same spell slots, they will remain with the exact same power they had when you first learned those spells.
While metamagic feats are still a thing, those known for causing balance issues, like empower or maximize, are gone. Quicken, a major offender, is still in the game, but is limited to once per day, will actually reduce the number of actions required to cast the spell by one, and using a free action, you will not be able to combine it with other metamagics. We don't know if those will be back (as we know them) in future supplements though.
Now, regarding the action economy, the game is far more balanced now. Weaker spells cost an action, strong spells cost three actions, and everything else is pretty much in the middle at two actions. Meanwhile, martial abilities are mostly one action, sometimes two, which will have some added combat utility to it. A single cantrip or magic missile will cost a caster two actions, while a martial can usually make two attacks with the same two actions.
All characters begin the game with three actions, which can be used to move, make an attack, switch weapons and so on. This helped reducing the reliance on iterative attacks that caused so many headaches to martials. Instead of granting more attacks as they level up, their attacks are better, the penalties for additional attacks are reduced and they can apply debuffs with those attacks.
We also have additional combat effects baked into weapons, such as being able to cleave when you critical hit with an axe should your character be specialized on axes. Or even simpler abilities like reduced penalties when making multiple attacks with that weapon (agile), or a bonus when attacking an adjacent enemy (sweep), or even a bonus for making multiple attacks in a sequence (forceful).
Another problem that plagued martials and didn't really bother casters was the fact that it was much harder to upgrade two or three different ability scores while casters could focus all their attention at one ability. The mandatory magic items required for all characters are completely gone, and while you may still have items that grant +X to certain skills, and others that grant either +2 or X value to a certain ability score, those items are no longer necessary to keep your character balanced against enemies. Bonuses to saves, AC, and attacks will happen naturally as you level up, allowing characters to focus on utility magic items in those body slots. The dreaded cloak of resistance and amulet of natural armor are gone.
A Belt of Giant Strength, for instance, will grant you bonuses to athletic checks (+4), bonus to break objects, grapple or get out of a grapple (+2). And finally, if you invest your Resonance (a resource used to fuel magic items with a limited daily pool), you can either get +2 to strength or get a strength score of 18, whichever is higher. Additionally, you may be able to use your reaction to grab a rock thrown at you.
Ability Score Boosts
All those +X bonuses that before came from magic items are now granted as you level up. Each character will gain four different ability score boosts, which are translated as a +2 bonus to a chosen ability if it's not yet at 18+, or a +1 bonus to that ability if it's already at 18+. So, even if the wizard does focus on a single ability, they will have three other abilities that they will be upgrading at the same time, just like the fighter. So, it's possible to have high ability scores on all necessary abilities even if you have no access to magic items at all.
Those boosts are granted every 5 levels to every class, meaning that the game will try balance itself in 5-level increments, expecting all classes to have roughly the same numbers regarding their attack rolls, defenses and skill bonuses.
But as this fine answer explains, it's still too early to have any definitive answer on this subject, especially since they are still playtesting mechanics, and even things like core mechanics (like how alchemists work) are being updated weekly.