They have written this about the Combahee River Collective:

The Combahee River Collective, a prominent group of Black lesbian feminists active in Boston from 1974 to 1980, strongly opposed political, moral, and medical discourses about “biological maleness” or “biological femaleness” due to these discourses’ simultaneously racist, sexist, homophobic, and classist implications and histories. In 1978, the collective wrote the famed Combahee River Collective Statement to clarify and define their political views and to offer a broad, inclusive vision for the feminist movement. In the statement, biological sex is clear target of analysis and conceptual un-doing. The Collective states, “We know that there is such a thing as racial-sexual oppression which is neither solely racial nor solely sexual.” Combahee further stresses, “we do not have the misguided notion that it is their maleness per se—i.e., their biological maleness—that makes [men] what they are. As Black women we find any type of biological determinism a particularly dangerous and reactionary basis upon which to build a politic.”

I would interpret this quote as being first about intersectionality - there are multiple interlocking oppressions. Women of colour are constrained by both sexism and racism. How does the author of this piece think sexism works if there is no biological sex hmmn?

Then there's the second bit which seems to me to be along the same likes as Simone de Beauvoir's "One is not born, but rather becomes, a woman." I am unsure if TRAs deliberately misinterpret these critiques of socialisation or if legitimately they just don't understand.

I also found this on the Wikipedia page for the collective:

" We are particularly committed to working on those struggles in which race, sex, and class are simultaneous factors in oppression."

Looks like the Combahee River collective actually did believe in biological sex, in fact it was their mission to dismantle the oppression they faced, partly because of the existence of biological sex...