It’s much more regressive to define women by adherence to or identification with gender norms.
The only thing all women have in common is being born female. Naturally, as that’s the only common factor between all women, and as it excludes anyone male, transwomen can’t accept it as a defining factor, so they try to assert that biology is regressive and gender progressive.
Women globally are oppressed on the grounds of our reproductive role, aka biology, aka being the gestating parent. This applies to women and girls who do, and those who don’t actually have children, simply because of biology.
Gender, OTOH, varies by culture, geography, and time. There are no “universally female” cultural expressions of gender, so trying to define anyone as either men or women by the extent to which they conform to or identify with any particular cultural gender norms is ridiculous.
Anyone can express themselves in any terms regarding gender, but gender identity doesn’t equate to sex, and it’s sex which actually matters.