Once again we have a clear example of how people don't actually believe the things they've convinced themselves they believe.
Now if you ACTUALLY believed that gender had nothing to do with sex you wouldn't formulate a sentence like "people with breasts and men" because you would "know", in the same way you know two plus two, that of course a man can have breasts, I worked with a transman who made no effort to disguise theirs.
Instead they managed to produce a statement which is arguably "transphobic" as it implies that a man with breasts can't be a man.
You can't actually "unknow" what sex is any more than you can unknow basic maths or the words of your first language. I suspect there's a mathematical law somewhere that someone can only use the "correct" language about gender for a certain time before accidentally revealing this.