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?