Self-serving pretzel logic with no truth to it.
Yes, XY/XX is a binary (if you ignore the unknown percentage of XYY/XXXXX/other duplications). However, that is only a neat and clean delineation for the fertilized egg, the single cell. Once that cell starts dividing to eventually form an adult, with tons of cells of different specializations, all biologists know that biology is messy. Chemical gradients within the developing clump of cells interact with chemicals present in the womb and later the mother's blood direct the expression of the genotype into the externally visible phenotype, and any of a million ways that process can go imperfectly without destroying the developing life.
That phenotype of sex is a spectrum, from complete destruction of the system (as both sexes will develop to a point in each embryo) of the other sex's presence all the way through imperfect destruction of the other sexual system to both sexual setups existing in the same phenotype. We see each other's phenotypes, judge them as being more or less male or female, and build a gender framework of behaviors, abilities, and attitudes on top of what we've judged a individual's sex to be.
As to the snark towards that moderator, of course people who insist all others must conform to the currently binary gender dynamic are conflating sex and gender. Sex comes before gender, and gender uses sex as its foundation to build its entire structure onto, but gender has no control over sex, sexes exist without gender. We have created entire gender concepts based on the phenotype expression of sex, and there are people attempting to impose those gender constructs onto everyone whether they fit or not.
Consider the person who was born with no external genitalia and one internal testicle, genotype XY, who was administered estrogen at the proper age to force female secondary sex characteristics. By genes this person is male, and should be a man, though who all who physically observe see only a fully female woman. Which of the currently binary genders should be imposed on this individual? It is the very existence of fringe cases that can't be accounted for in these rigid structures that show how untenable the constructs are.