OK I misremembered that part. It makes sense that after suffering trillions of losses the Culture would take steps to prevent the Idirans from doing it again.

And by "us" I meant fleshy meatbags, as opposed to Minds. Although in Excession he does raise the issue that there might be "psychotic" Minds. Gray Area's heart(?) is in the right place but it's easy to imagine them becoming a vigilante and pre-emptively nuking an especially annoying civilization.