(*)No matter what you self-identify as, I’m pretty sure medication and diseases and such only care about what your biological sex is…
Not quite; only to an extent. As Spacewyrm said here, quite a few things can be changed. Hormone replacement therapy does interesting things to your body; it can increase risks of certain diseases and conditions, and can even reduce risks from a few. ‘Biological sex’ is not as immutable as many people think, unless by it you strictly mean “the body/sex one was born as”.
Also, how “legitimate” a question one’s sex is is, well, debatable. I’d say that the one place where it definitely is a relevant factor would indeed be a medical setting*. Otherwise? Not so much.
*which seems to be the implication one of the TERFs above made (but I didn’t check what they were referencing, so that might not be the case).