Billy doesn't even know what the difference between boys and girls is, accourding to his very own comics. Because nothing is male or female ever, aside from the word and people saying that word. In Billy world, gender is a meaningless term that can be used whenever you happen to "feel" like a "boy" or a "girl".
So I think, any point he has to make about it is a stupid point. But I can try to take it apart anyway.
"If you identify as woman, then you were socialized as female."
I wouldn't say that is true. You get certain things told, but you are only told how a female acts or should act and how they should look. Same with boys. But there are just as much "femme" boys as there are "masculine" girls. And they still identify perfectly fine as what they are. One has little to nothing to do with the other in the long run.
"The way trans women look when they are children doesn't keep them from being affected by the messages society sends about women."
Why only women, actually? Doesn't this count for males too? Anyhow, I guess I can agree on this, but I will point out that Billy himself says often enough, that you can not tell gender by looking at a person, so saying "how they look" is stupid. The problem he can not use any other term for this however, is because he does not know what gender actually is made up of.
"Even when we're perceived as another gender than the one we actually are, we still internalize the messages about femininity and girlhood."
Aaaand if that were true, you'd know you are actually a male. Because femininity is usually connected to.. oh, I dunno.. . vaginas? Boobs? Having babies? Being able to become pregnant? Making sandwiches?