My thinking is that it underlines how much transphobia is actually an attack on women, as a whole.
The moment a woman steps outside the list of activities that these groups deem acceptable for women, or looks different than their standards of beauty for women, she is attacked as "not really a woman."
It's fundamentally the same as the attacks "if you were a real woman you'd stay home with your kids, not take a man's job" from *those* bigots. (Same group, mostly.)