I'm not sure what he meant, although I can infer enough to understand what he intended to say. If I wanted or needed to know more, it makes more sense to just ask instead of snapping at him for imprecise wording.
Most of us grew up in a world of binary gender. The forms said "Male" or "Female". Even in healthcare, it was a major change to get "Undetermined" added even though indeterminate gender is very common in newborns.
The idea of fluid gender is a much different concept for most people. And honestly, the categories of gender and sexuality have gotten to be so numerous that even those of us who are trying to understand are getting frustrated.
For example, you just gave a different definition of gender than the
Human Rights Campaign advocates.