I suppose it's also from years of their friends, family and allies playing along and using female pronouns, "woman" etc. They all end up actually believing it.
Language, as we (and they) know, is so important.
The co-working revealing that she knows he's a man "literally shatters the foundations of the relationship". A relationship in which he just expected her to believe his lie. I don't suppose he ever actually asked her if she was did, or if she was ok with the pretence.
"It just shows that they don't see me as a woman despite being nice to me. It ends up feeling performative."
Is he saying that if we know he's a man we should be horrible to him? Because all of my relationships with trans-identifying males are that I am nice to them, but I know damn well they are men.