there are people who claim they are the gender (not sex, because you can't change sex) opposite to what they were born as.
This point is Moot with a capital M.
You say 'you can't change sex' so it's gender that transpeople claim to have changed, not sex.
But for years the opposite has been true: TWAW, 'Transwomen are women. Transmen or men.' was pushed as a statement of fact, transwomen changed sex so that they were now no less a woman than you or me.
It was was transphobic hate speech to suggest otherwise.
Recently, perhaps because the idea that a human can change sex hasn't sold very well in the marketplace of public opinion, the idea of acknowledging that they remain men or women, but they now 'present' as a different gender, is more noticeable in trans posts on here.
One consequence of this is that as they acknowledge they haven't changed sex, it destroys their justification for using the wrong sex-segregated facility which are segregated by biological sex.
It may also mean that a whole generation of children will have been taught - partly through at school, partly from books - that humans can change sex and that transwomen are women and transmen are men. But by that time, even TRAs may have backtracked on that, claiming instead that as it's impossible to change sex, they have only changed that amorphous concept 'gender'.
I suppose the lesson to be learnt is not to rush to teach new and scientifically dodgy ideas as settled fact to children, because those ideas may not have a long shelf-life when exposed to reasoned argument.