You have a valid point. He is clearly using gender to mean sex. 2 years ago that was a legitimate distinction. The meaning of gender has shifted in just the way he describes as the very purpose of progressive deconstruction. A word that meant something very specific just a few years ago means something completely opposite today. That is the very case he is making in this article.
I agree that he’s using gender to mean sex. However, he’s also using gender to mean gender. Maybe he’s pursuing deep irony, but unfortunately, once we surrender the distinction between biology and society, we’ve got no argument against 51 “genders.”