Stephen doesn't want to have this argument. He doesn't want to keep bringing up seditious lines of thinking while lying to Cyrus's face about not being seditious himself.

He groans and takes another drink of wine.

"I'm not a traitor, Cyrus," Stephen sighs, as if the very idea is ridiculous. It is, of course. Stephen knows Cyrus believes that Stephen isn't a traitor, and Stephen has absolutely no convincing to do. The most natural thing is to treat the idea as a joke.