What does this even mean? Like, how can someone not be "legitimate"? Does it mean "illegal" as in illegal alien? Is anyone claiming that trans citizens should be deported? Or does it mean "imaginary"? Trans people obviously exist.
JKRowling is simply pointing out that the word "woman", which used to have the clear and simple meaning of "female human" (or sometimes "adult female human"), was now not only redefined & politicized to basically mean "opinion" (or "a woman is someone who thinks they're a woman" which is a clearly nonsensical recursive definition) and is used for Orwellian speech control.