Except they haven't. There are a small handful of examples, that people both greatly exaggerate, and repeat over and over again. Trans people have been allowed to compete for many years prior, and have not overrun women's sports.
Simple math is that even being a small minority, trans people will, occasionally, win things. Even if there are zero competitive advantages to being a transgender woman, some trans women are gonna excel. Finding a handful of examples of trans people being good at sports isn't actually proof of advantage.