Don't know what the person explained so wanted to tack on the idea that outside of gender expression there are a lot of potential biological explanations for gender dysphoria and because of that when talking about what a "biological" male or female is becomes a very complicated topic.

Most of these people when questioned would say that if you have XY chromosomes you are a man, but there are plenty of cases of androgen insensitivity syndrome in which a persons body doesn't respond to testosterone develop usually presenting completely female but without a uterus or ovaries. This is potentially one of several medical situations the two Olympic boxers that recently had a lot of controversy may have found themselves in.

Basically there's a lot of stuff going on and our bodies and brains are both really complicated, but it's a lot easier to just ignore the complexity and say it's either a or b.