Yes gender is a made up concept.

But sex is also not binary.

So ideas like biologically mailed are just that. Ideas with no real merit.

Biologically XY could be used. But without clear medical evidence. The BBC has zero idea if that applies. And people are known to be born female. To the po8nt of getting pregnant with xy chromosomes.

And while documented cases are rare. They are rare because it is rare that anyone is actually tested,. So how common is not really known.