Or do those who think "transwomen are women" just think it's a massive coincidence that women across all cultures and throughout all of history have been systematically disadvantaged with respect to men?
I agree with all of your post, PricklyBall, but this is the question that I've never, ever had a decent answer to, when debating with transactivists. It's either:
a) denial: yes, cis-women have historically been at a disadvantage, in more primitive cultures... but all that ended when women got contraception and the vote. Everything is completely fine now, and actual biology is irrelevant.
All modern-day women's issues= erased.
b) deflection: you think cis-women have it bad? Transwomen are the most vulnerable, disadvantaged and abused social group ever.
All modern-day women's issues= dismissed.
c) re-positioning: female biology is actually a form of cis-privilege, so we should take care to discuss all women's rights issues relating to biology with the appropriate tact. We shouldn't use trans-exclusionary language, even if that means erasing women from discussions about female biology (eg an article about denial of endometriosis treatment being criticised for use of the word 'women', because 'not all women have wombs, and not all who have wombs are women'; an article about menstruation being criticised for the same, with the suggestion of substituting 'period-havers' for 'women').
All modern-day women's issues: tactful discussion only, please. Clearly and openly discussing women's health is less important than the feelings of those do not experience these issues.
d) personal attack: transphobe! why are you so obsessed with genitals! educate yourself!
All modern-day women's issues: just shut up.
In every case, the answer has involved women shutting the fuck up about their history and their rights. That's exactly why a transwoman being appointed Women's Officer is so concerning, and is more than a token right-on gesture to inclusivity.