Disregarding the rest, transwomen are not all born with typical XY chromosomes and the XX/XY karyotypes for sex designation are a lot more complicated than xx=lady and xy=guy.

Aside from extra or damaged chromosomes, chimerism, and other genetic differences, there are mitigating factors in genetic expression that can affect development in a number of ways. As one example, there are women with XY chromosomes who don’t even know it, and recent studies have found a lot more of them than previously thought to exist.

So, even before taking into account gender dysphoria, sex isn’t that cut and dry in the first place.