> The problem for this argument is that there is no actual data that trans kids and specifically trans girls are any better at sport than other girls.

There is considerable evidence that they aren't. But that's not really relevant, because you have to remember segregation in sport has never been about competitive fairness, it has always been about allowing those who are socially superior to avoid the embarrassment of having to compete in an environment where they might be defeated by their social inferiors.

It is why women were long banned from competitions, and then shortly after exclusion seemed to harsh for evolving attitudes, they were segregated from men. And it is why trans people are being excluded from competition now. It's why racial segregation in sport was a thing. When competitive fairness is raised as an argument for segregation, it is pretextual, not the real reason, so counterevidence is irrelevant.