A second anecdote related to that episode, this time not one I was personally present for, but told of by another fan:
Ellison had a well-deserved reputation for not suffering fools gladly. As Guest of Honor at another early con, he talked about the writing and production of
City. During the Q and A, a guy got up and took issue with the idea of "writing" Star Trek, insisting the series was so uniquely great that the actors spontaneously improvised all the dialog.
My friend didn't repeat Ellison's white-hot reply, so you'll have to use your imagination.