Jack Posobiec posted a picture of a celebrity trans man, pre-transition — the trans man already had a famous career in the days when he presented as a woman — and Posobiec commented: "Remember what they took from us".
It's plainly sexist. The implication is that women (or people who look like women) belong to men; the illusion and spectacle of femininity belongs to men. When a trans man (or, perhaps, a woman) begins presenting in a more conventionally masculine way, instead of seeing a person who is self-actualizing, there is a collective "us" from whom the feminine image has been robbed.
Rhetorically, it echoes the form of Nazi antisemitism. There is a "they" who is allegedly stealing some kind of cultural purity from "us." As a matter of Posobiec's personal history, "Posobiec targeted Jewish journalists with antisemitic hate, as Hatewatch detailed in a story about his links to the white supremacist movement." (Southern Poverty Law Center, July 8, 2020) And from the Southern Poverty Law Center's profile page on him: "He has also collaborated with white nationalists, antigovernment extremists, members of the Proud Boys, and neo-Nazis in his capacity as an operative." So the language is related. It doesn't come from nowhere.
Here's a screencap posted to Twitter, which also has a URL to Posobiec's original July 5, 2022 post on Telegram if you have to see the original for yourself.
A reminder: Transphobia is sexist. And also linked to antisemitism and fascism.