It is a mistake to see it as a 'joining' of womanhood. It isn't. There is no sisterly feeling apparent in the tide of articles and social media posts and behaviours at rallies.
It's about taking womanhood from women, breaking it down for parts, using and colonising the thrilling and fun bits, and throwing the grotty bits out of womanhood along with female women and leaving that as an unnamed lower caste that are irrelevant and not to draw attention to themselves.
It's taken years to get to that realisation: I started with the female socialised good will and willingness to listen and empathise and try to understand (see Chumplady for details on how women will waste years trying to understand their abuser and what they do what they do, and providing them headspace when their abuser will never spend a moment reciprocally considering them-it isn't like if you just understand enough they'll quit abusing or you will stop minding so much.)
I was considerably helped along though by the first male women's officer for Labour. Ok I thought, innocently, this will be interesting, I'm open to seeing how a male person can fully engage with and represent the needs and issues of women from their position of care and interest in womanhood.
Oh lord the naivety.
Of course that women's officer promptly announced that anything that did not also equally apply to them as a male was not a 'women's issue' because transphobia/excluding males from womanhood, and those subjects (you know, about 99% of them) were not women's issues, nothing to do with them, hell fire and damnation cast upon any woman who mentioned them, and for 'women' read 'male with identity'.
Womanhood is not a reward for suffering (by looking like a woman, with all that humiliation involved for a male, which is just lovely if you're female and to look like you is seen as a humiliation, or a caricature of mocking a la Dylan Mulvaney). It's not a prize women can be mugged for. It either is at birth or it isn't. Head patting male people and telling them that yes, yes, absolutely you see them totally as women is not kind to anyone. Male people can be transwomen, great, good on them.
But women are not transwomen.