Even if this AoE was considered attack when used this way (which it's not: no attack roll, and see other answers), note that Redirect Attack still wouldn't apply.
When a creature the goblin can see targets it with an attack
The target of the breath weapon is a 30-foot line, not a "number of creatures within <some restriction>". Creatures in the area aren't targets of the dragonborn directly, only of the magical effect created by the dragonborn. An answer on What counts as a target for a spell? goes into detail about the two usages of "target" in 5e rules, one being the technical term that ties into the spell targeting rules (line of effect / clear path to target), and the other broader English meaning that can include being affected by an AoE. (Fireball's spell description even uses the word "target" to describe creatures in the area, separate from the point-within-range targeting part.)
The Redirect Attack wording is somewhat similar to Sanctuary (1st), except that Sanctuary also includes being targeted by a creature with a harmful spell in general, not just an attack.
You ward a creature within range against attack. Until the spell ends, any creature who targets the warded creature with an attack or a harmful spell must first make a Wisdom saving throw. [...]
This spell doesn't protect the warded creature from area effects, such as the explosion of a fireball.
This is part of one spell's description, so it not covering AoEs doesn't necessarily mean that the same wording elsewhere should be parsed the same way. But it does support that interpretation.
The dragonborn targets an area with their breath
The target of a spell like Lightning Bolt or Fireball, or a breath weapon, is an area. A creature in that area isn't a target of the ability or spell in the technical terminology of 5e; a spell would need a line of effect / clear path to target. (Non-spell things might have different targeting rules.)
Compare spells like Word of Radiance: Each creature of your choice that you can see within range must succeed on a Constitution saving throw or take 1d6 radiant damage. - you see and target those creatures.
Chain Lightning (7th) makes it even more explicit because its target can be a creature or object, so it uses "target" as a placeholder to define later: You create a bolt of lightning that arcs toward a target of your choice that you can see within range. Three bolts then leap from that target to as many as three other targets, each of which must be within 30 feet of the first target. A target can be a creature or an object and can be targeted by only one of the bolts.
When the target is an area, the thing you need a line-of-effect to is some part of the area, not to each creature in the area. e.g. for Shatter, A sudden loud ringing noise, painfully intense, erupts from a point of your choice within range. The stuff about "each creature" within 10 ft of that point making a save comes later.
Some things are less explicit about separating the choosing of area from the effect on creatures within it, like your breath weapon, or like Burning Hands (1st): Each creature in a 15-foot cone must make a Dexterity saving throw. But Burning Hands lists Range: Self (15-foot cone) which makes it explicit that you aren't targeting creatures in the area, the target of the spell is yourself.
I've mostly picked spells as examples, not other non-spell class abilities, but the principle is the same. For example, Summon Wildfire Spirit: The spirit appears in an unoccupied space of your choice that you can see within 30 feet of you. Each creature within 10 feet of the spirit (other than you) when it appears must succeed on a Dexterity saving throw against your spell save DC or take 2d6 fire damage.
Sanctuary wouldn't stop you from doing this because it's not an attack, and doesn't target them directly. (Also because it's not a "harmful spell"; that wording in Sanctuary excludes even single-target class abilities like Ancients Paladin channel divinity: Nature's Wrath, a single-target save or restrain effect.)
Attacks always target creature or objects
In general, all attacks have a creature or object as a target. So do some non-attack things like Sacred Flame (target a single creature, save or take damage), but AoEs like fireball or lightning bolt don't target individual creatures or objects.
An attack involves an attack roll against each targeted creature. (Or it's a special attack like a grapple that involves contested ability check.)
Replacing an attack in the Attack action isn't sufficient to be an attack
If you were a bladesinger and replaced one of your attacks with a saving-throw cantrip like Toll the Dead, it wouldn't be an attack. You're giving up one of your attacks from the Attack action to do something else, not doing something else as an attack.
It doesn't matter if the cantrip deals damage or not, you're still casting a cantrip instead of making an attack. If the cantrip itself involves a spell attack or weapon attack, then it's also an attack for that reason. Otherwise it's not, e.g. an AoE or single-target saving throw cantrip, or non-damaging like Dancing Lights or Minor Illusion.
(The bladesinger rules wording doesn't require it to have a cast time of 1 action, so Mending is possible instead of the normal 1 minute?)
Same thing applies for this breath weapon: it's a non-attack thing you can do instead of one of your attacks when you take the Attack action. It targets an area, and has no attack roll.
Compare with grapple and shove
Grapple and shove explicitly are special types of attacks, because they say so in their wording:
When you want to grab a creature or wrestle with it, you can use the Attack Action to make a Special melee attack, a grapple. If you’re able to make multiple ATTACKS with the Attack Action, this Attack replaces one of them.
It's an attack because the rules say it's a "Special melee attack", not because it's replacing one of your attacks in the Attack action.
