When an nb-identifying female claims that she is “not a woman” for whatever reason, she is making a bold ass assumption about every single female human being to have ever existed on Earth
They are? And what assumption do you think they're making here?
something invariably sexist.
How so? "It's sexist, trust me" isn't an argument.
For her to not be one, she must be different from all women.
Yes. That difference is that they lack a sense of gender/sex that tells them they are female, or possess a sense of gender/sex that tells them they aren't female, but also not male. None of this makes sense to you because you deny the idea that anyone posseses a sense of sex/gender, despite the fact that it is a well documented phenomenon. I mean, why not just listen to actual non-binary/trans people instead of assuming you know better than us how we feel?
Like ok, if we are all the masters of our own identities and can be whatever we want, why are trans activists pressed over women choosing to call ourselves adult human females?
Because this isn't about women choosing to call themselves "adult human females". Nobody cares if you call yourself an adult human female. This is about you insisting that "woman" and "adult human female" are perfect synonyms and that that means trans women aren't actually women.
Why must we be labeled “cis”?
The same reason people attracted to the opposite sex are labeled "heterosexual" or "straight".
If you don’t want to be identified by sex why must we be by “gender”?
I might consider taking this argument seriously if it came from someone who knew what the terms "sex" and "gender" mean.