The argument ends up being reductive on both sides.
At the high school level is probably where competitive disadvantage is going to be the strongest because a trans high schooler has had relatively little (or no) time on HRT. So from a competitive standpoint, it's not going to be fair. If that's all you care about it's sensible to ban her. I'd argue there's nuance here because sports aren't just about competition, especially at that level, and personally the medals and awards I got from racing are far less important to me than the character building experiences and friendships the sport has brought me, but I absolutely concede this is unfair. Doesn't mean transphobes should be losing their mind at her personally. They should be going after OSAA.
College is different, pros even moreso, because there are basically no studies on experienced athletes that track them during their transition to measure the loss of performance from HRT. There are a couple of okay ones on untrained trans women over long periods of time that indicate their strength regressed to the point that their measurements line up with average cis women and whatever advantage they have is basically the difference in weight (and you'd obviously expect a 5'10" woman to be stronger in most metrics than a 5'4" woman, cis or trans). Competitive advantage for high levels of sport is still pretty much unstudied and likely specific to different sports.
So the people you're mashing together for the sake of argument here are probably not the same people, are probably not talking about the same things, and probably have a more nuanced opinion than you're giving them credit for.