"This is just bullshit, isn't it?"
It is bullshit. In order to believe it you have to subscribe to a quite complex theory. Postmodernism isn't something which I would say is obvious or intuitive. So the theory is that you don't have direct access to reality, what the world is really like. The world ONLY makes sense to us through 'discourse', which is writing, talking, using language to discuss reality. Reality is always filtered through language.
There is no such thing as objective 'truth'. Rather, there are just competing discourses --competing theories about the way the world is. None of these theories actually has a claim to describing reality as it really is, because we simply don't have access to that. And these discourses compete based on power. Dominant discourses are the most powerful. you might think that science is true, because it get things right about the world (we have planes, medicine, covid vaccines) but in fact science is just one discourse among many. And it's a powerful one because it's so widely accepted. But that doesn't make it true.
What this means is that the decision about what disclosure to follow is always a political choice based on power. Believing in science is just as political as believing in Christianity. This means we can choose what discourses to follow, much like we can choose what political party to vote for. The TRA theory is that choosing to follow the science disclosure, that there are two biological sexes, is oppressive because it leaves trans women out of the category of woman which they would like to be a part of and also leads to oppression of trans women. It would be a more just, fairer world if we instead adopted the discourse that trans women really are women, and women is a social construct, so we should abandon the scientific view. Stock's error, according to these guys, is thinking that science reflects reality when in fact believing in science is a political choice much like believing in the Tory party.
There are obviously massive errors with this. It's pretty hard to swallow that science, the field that has given us technology and improved our lives immeasurably, is just another 'discourse' among many others like creationism, flat earth theory etc, and that it doesn't actually reflect anything real or true about the world. Quite simply, science works. Believing in crystals and woo doesn't.
Secondly, postmodernism is committed to the idea that there is no objective morality. There is nothing but discourses butting against each other and competing for power. There is no such thing as a 'good' discourse compared to an 'evil' one. So it is difficult to know what exactly they are getting at when they talk about making a political choice to accept trans women as women in the name of justice. What is 'just' or 'morally right' is nothing more than another discourse. There is no objective reason to prefer the 'Trans women are women' model over the 'science' model because you can't point to one discourse being objectively more moral than another. This is to import something objective and real into postmodernism, which postmodernists deny. Note that Butler , the dominant postmodern theorist around today, really struggles to expunge references to morality from her theory. She seems to think the invasion of Gaza is objectively bad and not just a matter of opinion or taste.
It's all bollocks.