To be fair, he is correct that it's insane. Why reject the chance to cause an AGP's kidneys to fail from beyond the grave?
I think it's mostly just reverse-engineered from the primordial untouchable belief that these people must conform to in order to continue to be accepted by their tribe (Trans Women Are Women). If you admit that genes relating to biological sex might influence behavior, then you might have to admit that male genes might cause males to behave differently from women, and that causes cognitive dissonance related to the TWAW idea. Therefore, men are not influenced by their genes.
On the other hand, I doubt this kind of thinking could have taken hold if there was not already fertile ground for it: American society in general (not just liberalism) relies heavily on the idea of individualism and self-reliance and merit, and the idea of people being influenced by their genes (and therefore potentially limited by their genes) does not sit well with that mythos. Fredrik DeBoer talks a lot about this and gets shunned by the left for it, even though he's a leftist. But I think most right-wing Americans think similarly, even if they use it to justify different things. The idea of the infinite-potential blank-slate individual is so pervasive in American culture that most of us don't even notice it.