I spoke to a football coach personally about this. BYU has a matrix that they use for bringing in athletes.
If a recruit scores below a certain level on the ACT and with his high school grades, Bronco must write a letter to the admission office promising the athletic department will do all they can to make sure this person will succeed. The admission office will accept these recruits only if Bronco writes the letter. I knew a kid who did not get accepted to BYUm, but Bronco wrote the letter and he got in.
If the recruit is too far below the level BYU has created, then even a letter from Bronco will not get that person in.
The football coaches will eliminate some recruits because they know they cannot get them in no matter what, so these recruits go to Utah:)

So, not every P5 football could get into to BYU, yes, many could, but you said every player that plays for a P5 school would get in on athletic merit alone and that is dead wrong.