I agree which is why I phrased my answer the way I did, but I don't think the claim is really analogous to "2 sexes, male and female. Simple and true." (not that you claimed that, just what someone above us said). The equivalent claim would be something like "humans are a sexually dimorphic species" which I think is of course true.

All to say, I don't really care much about these claims in isolation, I care when someone says e.g. "there are two sexes, so we shouldn't let trans people transition". Those conversations are a context where edge cases, caveats, and complexities all play a huge role.

Agreed.