what they really mean is that “men are supposed to be one way and women are supposed to be another,” with the implication that someone isn’t a real man or women if they are not that stereotype
you can fit the stereotypes for your sex but be trans
The word “stereotype” is carrying a lot of weight here and it’s sort of ambiguous what it means.But for example, there are trans men who are feminine and wear dresses and such, and there are trans women who are masculine/butch and wear pants and suits, etc.They’re still trans because they identify as a gender different from their sex, and any of them may pursue different forms of physical transition or not at all.