> stating that the biological sex exists and in human case there are two sexes is sexist.
You got it! Sort of. It's not a problem to use the concept of biological sex, but it's completely wrong to say that humans have two biological sexes. The main problem is in the assignment of chromosomal configurations to sexual configurations; humans have a dozen or so intersex [0] configurations which don't fit neatly into binary gender theory.
And this has real-world consequences. There are many countries which have canonicalized various sorts of human-rights abuses based on the binary theory, mostly by establishing some sort of concentration camps for homosexuals. The author's writing forms a small but real brick in the wall of these camps.
> 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.
This is the error. It doesn't make sense to criminalize sexism, and indeed it's not criminalized in any jurisdiction that I know of. Instead, what's criminalized are specific deleterious pragmatic aspects of sexism: Sexual harassment, genital mutilation, conversion therapy, forced emancipation of trans children, human trafficking. When biologists are sexists, they are usually being reductionist as the author is doing, by minimizing and ignoring the diversity of the real world's biosphere; they are not directly advocating for the harm that is done by other, more hateful people, based on their reasoning.
Also, yes, my priors are that people are 99% racist, 80% sexist, 85% delusionally religious, and 20% fascist; this gives me a weak but steady belief that the typical person I talk to is, in some way, a sincere believer in deeply harmful and falsifiable cultural beliefs. I'm in the USA, for what it's worth; hopefully it's not like this everywhere.