Hmm, that's interesting. I think there are TERFs on both sides of that debate. They just both use mental gymnastics to make it support their transphobic ideas. I've definitely encountered a ton of TERFs who claim that being trans is not legitimate because sex and gender are both 100% fixed at birth and therefore if you were born "male" you'll always have a "male brain," and if you were born "female" you'll always have a "female brain and there's nothing you can do to change it because it's innate. Those are the biological essentialist TERFs. But I've also encountered a few of the TERFs you mentioned, who believe that sex is a social construct and that "therefore transitioning from one sex to another isn't valid." But that logic makes so little sense to me that it makes my brain hurt. Wouldn't that make cisgender identities equally invalid?