The specific rule about casting two spells on your turn involves actions and bonus actions, not reactions. No broken rule there.
And he's not interrupting the casting of his fireball. He's completed it. Counterspell doesn't prevent you from completing the casting, it stops the magic from taking place. The fireball is completed, the first counterspell is completed, the second counterspell is completed and prevents the first from canceling the magic--not interrupting the casting, canceling the magic--of the fireball.
You can houserule as you like, of course, but the Sage Advice is entirely within both the letter and spirit of the RAW.
The caster has to stop casting Fireball to cast Counterspell, then complete Fireball. If Fireball had already been completed then enemy Counterspell wouldn't work. It's definitely against the spirit IMO and is a bad ruling.