I think there is another thread on this article.
I don't think the problem is just that he dismissed sex as real. His ideas about gender, how it's structured, etc - a lot of that wasn't rigorous either. It's made up explanations that seemed right, and then he works backwards to "prove" that they are valid. Sometimes intuitions like that pan out, but you have to do the work to show it from the ground up, without biasing the work, which can be hard.
What's also interesting is that while he says that at some level he knew that he was on shaky ground it doesn't seem like he was being directly dishonest It's what was going on all around him in gender studies, and IME it goes on in many areas of study that deal with these kinds of political question. When the university is training young academics to think this is how it is done, of course that's what they will do.
I've wondered if studies areas are actually a good place for undergraduates, or if they should maybe strictly belong to graduate studies, while undergrads get a foundation in history, biology, etc.