Does casting Counterspell on one's own turn contradict the '1-non-cantrip-spell per turn' rule?

Question

I've read both the question Casting multiple spells in a single round and the question Is it possible to cast multiple spells per turn? about using multiple spells on my turn. I know there are plenty of question concerning counterspell and how it works on my own turn.

From the example in Sage Advice Compendium, I also know that I can counter a counterspell that was cast against me on my turn:

Can you also cast a reaction spell on your turn? You sure can! Here’s a common way for it to happen: Cornelius the wizard is casting fireball on his turn, and his foe casts counterspell on him. Cornelius has counterspell prepared, so he uses his reaction to cast it and break his foe’s counterspell before it can stop fireball.

However, from my understanding, that contradicts the PHB:

You can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action.

Both fireball and counterspell are not cantrips, and both are cast on the same turn. So is the example in the compendium misleading here, or do I have some gap in my understanding?

Answer 1

You may counterspell on your turn, as long as you have not already cast another spell with a casting time of 1 Bonus Action.

The rule you quote that you say is a contradiction is the "Bonus Action" spell rule. The full rule is:

A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven't already taken a bonus action this turn. You can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action.

If you have already cast a spell using your Bonus Action, you cannot cast any other spells that turn except for a cantrip with a casting time of 1 Action. This means you may not cast counterspell if you already cast a bonus action spell. However, if you have not cast a Bonus Action spell yet, you are free to use your Reaction to cast counterspell.

Since fireball has a casting time of 1 Action, it does not fall afoul of the Bonus Action casting rule.