If HYPSM reject, say, 65% of students w/ perfect GPAs and SAT/ACT scores, that means 35% get in. That's good odds, no?

Right. What they would need to show is that all things being equal, what is the boost for being a legacy? Is it merely a tie breaker between equally rated students or is it something that can vault a less highly rated student over others? Or, is it that legacies are often exceptionally well qualified with stats better than or equal to the rest of the applicant pool.