That comment is vague enough that I’m not going to bother guessing what you mean, but the reification fallacy is a common trap for fascists, whose ideology depends upon making their preferred version of reality the “correct” one.

If sex is biological, then we as humans can only observe it secondhand via complicated and potentially ambiguous medical tests. Our direct experience only shows us what someone looks and sounds like, which can also be ambiguous.

In the Real Physical World, you can’t always tell. The people who don’t want to take that ambiguity seriously are trapped in an artificial world of language where they value their own prescriptive ideals for what the words mean much higher than the reality that words were invented to describe.