If Borcherds accepts TAs then they work for him. The point was that he can do practically whatever he wants and doesn't have to worry about it harming him.

Thanks for explaining to me how the academic system I work in functions. Just like most managers don't literally pay their employees, the company does. The university pays TAs through funds from their various revenue streams, including endowment, tuition, grants to the university, and through indirect costs collected from research grants to researchers, probably including Borcherds own grants. So really a lot of different things pay the TAs, the university is just the clearing house.