Edge cases are still real cases. It’s a proof by counterexample against the binary gender claim, which states that there are only two genders – a single example not fitting the model can be sufficient to invalidate the model, that’s how science works.

And transgender identity isn’t nearly as common as cis gender identity either. If someone can accept chromosomal abnormalities as edge cases, which are by definition, still valid cases, why can’t they do the same with gender identity?