Huh, that does make a lot more sense. I guess I'd been assuming that any reference to someone "averting" a prophecy was actually just someone forcing the better branch of an EitherOrProphecy (tvtropes). Like if Trelawney had said "HE WHO WOULD TEAR APART THE VERY STARS IN HEAVEN IF NONE STAND AGAINST IT." The inference that prophecies don't always come true fits Quirrell's behavior much better.