TERFs don't really claim that biological sex is real. They claim only that gender is not. If the claim about gender is true, a person cannot "know" their gender innately, because there is nothing to know.

Trans people, if they are real, refute the TERF position, and so they must be "fake," i.e., mentally ill, and should be treated accordingly. Nothing personal, mind you, but the TERFs have a "gender is cultural" axe to grind, and trans people are standing in the way.

That some "gender critical" people also believe that sex is "real" or "binary" is really beside the point. Lots of people believe things for the wrong reasons. In this case, the force driving the anti-trans bus is the belief that transgenderism refutes gender criticalism. The rest is just noise.

[Update, 11/3/21 — Sex vs Gender — a Biological Location for Gender]