how does a y chromosome help if they never underwent a male puberty? they would just have the height, strength, etc, as a cis woman, since testosterone never had an effect on their body.

and what about trans women who are xx? should they be allowed to compete because they have xx chromosomes while someone who transitioned at 14 and has xy can't?

Olympians aren't Olympians just because they worked hard. Not everyone can reach their level of athleticism. Because they were born with advantages. And some people, like Michael Phelps, have absolutely massive advantages. So if genetic advantages are the problem, then we need to look at all genetic advantages to make things as fair as possible.