Quirrell might have been referring to his initial plan (as Voldemort) to end Harry's life back when Harry was a baby. Quirrell/Voldemort could not have known about a dark ritual then, because he hadn't created it.

I still don't think the "dark ritual" hypothesis is very strong, because I can't see why Quirrell would have done it deliberately, so it happened accidentally. It's not clear whether "accidental" dark rituals are even possible in the HPMor universe, but if they are, Quirrell ought to be more careful to avoid them.