it’s really just that binary sex is a construct. there’s a practically infinite number of bio sexes (one for each person), and the idea of male/female (and even male/female/intersex) is something we made up. the sexual characteristics of our bodies aren’t made up, but the categories are
exactly. only in the last ~80 years has "sex" taken on a "purely biological" definition, historically sex referred to all aspects of being a man or women. the definition is also highly contested and has shifted over time, ex. with the development of chromosomal analysis. (see image) this is not "naturally occurring", someone decided where exactly those lines fall on that ruler and it is (re)inforced by the medical profession through violence against those who fall outside it.
it’s not just that binary sex isn’t “true”, it’s that sex itself is a social construct. social construct doesn’t mean that something is “false” or “not real” or not based on “material reality”, it just means that something is understood thru shared concepts. time is also a social construct, yes it exists independent of human understanding, but as we understand it that is thru as a social construct (and why people relate to it differently between cultures, just like sex/gender).