Earlier quoted context omitted.
Probably going straight for the ad-hominem is a reason why people are bothered by your otherwise reasonable observation. Not everyone that's wrong is a "hack". If I remember correctly HN encourages discussions in the spirit of being charitable to another person's opinion.
We shouldn't be charitable to racism, sexism, delusional religious beliefs, or fascism; they are too harmful to the world. In this case, you're asking me to be charitable to a sexist article whose author does not justify their credentials beyond being a professor of philosophy; perhaps they are only accidentally sexist here? Seriously, do some close-reading. The very first sentence: > It’s uncontroversial among biolo…
If author justified his credentials with more titles, it would be better? If he was a biologist, his paper would stop being sexist in your opinion? Why does it matter?
As for the article itself, it isn't sexist IMHO unless you believe that stating that the biological sex exists and in human case there are two sexes is sexist. If that's so then the most biologists I've met are jest sexists and I should probably call the police and denounce them as petty criminals.
You're right that author contradicts himself in the sentence "The fact that no species has evolved more than two biological sexes is also a puzzle". It's definitely a logical error on the author's side and is plainly wrong, but it's not the main point of the paper.