I think their point is more that if gender is socialization, then why would anyone ever have a strong and persistent sense that they are a gender other than they one they were socialized as? Gender is socialization means you are what your gender is what you were raised as. The idea is that it was the way you were raised that makes you a boy or a girl. This view absolutely has problems accounting for trans people, since trans people are generally claiming to be something other than the gender they were assigned at birth and then raised as.