I agree that the percentiles don’t really matter at the upper levels, because a 800 SAT I math and 800 Math II doesn’t say anything about ability in proof-based math. It just says the student is good at math through AlgII/Trig.
There is no test HMC can use prior to admission to determine which kids will be good at proof-based math. So, if they want to diversify beyond admitting students from TJ in NoVa, some publics in Silicon Valley, some select magnets/privates and a few students who reached proof-based university courses in HS, they will have incoming students with a wider range of math abilities. And, they seem to be doing fine with that.