FWIW I've never met a qualified doctor or medical professional who would say there's only two genders in the biological(not social) sense of the word. There's a huge variety of medical conditions that exhibit physical or mental aspects of both socially-defined genders. There are people that you can't neatly class as male or female due to having both male & female reproductive organs and/or chromosomes. People with XY chromosomes that are female and vice versa, or people who have natural hormone imbalances that makes their body "grow" into a gender (physically) other than that they were born with.
Even completely ignoring transgender people or anything to do with them(Most trans people actually believe there are two genders and they fit into one of them, just not the one they were born with, so are irrelevant to this discussion entirely), I think it's still widely agreed in modern medicine that you can't neatly define two genders even if the vast vast majority of people would fit into neat definitions.
It's like saying "humans can't be over 7ft". Sure, you can say that's true in an evolutionary sense, but humans are biological so can have a near infinite number of mutations within their DNA that makes neatly defining any external trend in biology a fools errand.
To say otherwise is ignoring decades of medical research & world wide consensus in medicine, which is why the WHO have their guidelines.
I'd be far more worried about the fact many schools in right wing countries & states still teach creationism as a scientific theory as valid or moreso than evolution.