/That’s/ what frustrates me about this episode.
The Disir treat everything about Arthur as if it was a flaw, and insist that the only true way is theirs. They treat his honesty and conviction as if those were flaws as well.
The only thing they say to him that’s a true thing is “you persecute her followers”.
Also, when Merlin tells Arthur, “There can be no place for magic in Camelot,” I think Arthur knows. That Merlin has magic, and he was waiting for Merlin to finally be honest to him.