The group 'cis' women is probably a tiny one in reality, because the definition of 'cis' means someone whose abstract gender identity just happens to match her or his sex.
Most of us don't have the kind of abstract gendered soul that this definition implicitly uses, so most of us can't be 'cis' even if we never transitioned.
Forcing the term on us is like a religious sect which calls non-believers heathens demanding that all non-believers also call themselves 'heathens.' And when non-believers complain, they are told that all 'heathens' means is a non-believer of the tenets of that sect. Nothing to see here, move on.
This forces us to act as if the gender identity ideology is true and something every single person believes in.
Which is the point of the exercise, of course. It turns 'women' and 'men' into feelings-categories, and it turns female women into the privileged part of the new woman-as-feelings classification.
In this new category (created by an authoritarian fiat by a small minority which is busy erasing all references to the female sex class) trans women would, indeed, be a sub-section of 'women', and the more oppressed one, too.
But in that system we won't be able to address sex-based oppression very well, given that we are not allowed to even have a name for its victims.