The point is he was devaluing her. Specifically in a manner that has no male equivalent. That is sexist. The environment that allows for such an insult to be made is irrelevant to the fact that he actually made the insult. The fact that sexism exists in other media (which by the way you are entirely wrong about, movies, tv and books have been criticized similarly for decades) is irrelevant to the fact that he made the insult.Saying something along the lines of, "All you're good for is whoring out" or something is sexist.
That is basically exactly what he said. "Could you be considered nothing more than a glorified booth babe? You don't seem to add anything creative to the medium." That all her accomplishments are meaningless, and that all she does is sell sex to the extent that booth babes do.
What are her accomplishments in the medium? You listed the Guild, how has that moved gaming forward, has she created the next half life? She might have some influence and might have brought some change in the gaming medium, but has done little to actually progress it.