AFAIK all information anybody had at the time was that he grew up in a good gun-loving Republican family and he'd written some silly memes on the shell casings.

The discord chats and his relationship with a trans woman were AFAIK not revealed yet, or at least were so new that they maybe hadn't made it to Kimmel's writers room.

That kind of problem gets a demand of a retraction, not a firing.

Contrast that to a Fox News host calling for mass executions of homeless people the other day (and since that day there have been multiple mass killings of homeless people). That guy got off with a thin apology.