It might be true that the majority are coming out of top tech schools, I really have no way of knowing that. However, I'm skeptical about how important the school is to Google as an employer given their rigorous screening process. Coming from a top tech school certainly can't hurt but I don't believe the school you go to necessarily dictates what quality of engineer you are, though it can increase your opportunities for sure.

On the second point, I think interest prior to University is certainly helpful but saying it's a majority is just an anecdotal conjecture itself.