There may or may not be physical/biological sex differences that affect performance in tf2, but I think it probably has more to do with the fact that women in general tend to be less competitive than men, and that most of the women who play probably have less experience with video games in general than men.
BapeDo a fattest girl award, I would be on the edge of my seat.
It's ignorant things like that that help prevent women from putting much effort into playing video games. It's not exactly encouraging when you're constantly told you're bad and always will be because you're female, and people constantly assume that you're fat, only play medic, etc. I joke about it too but honestly it gets old after a while. Repeatedly saying things like women are bad at games tends to make them believe that, adopt that label, and it becomes a self-fulfilling prophecy.