I don't think it does. It assumes that people can pick up something useful from listening to someone who's perfected their art regardless of how good or bad they are at speaking about it, which might not be true, but I don't think it assumes anything about quality of the 'teacher'.

I would hope that Masterclass filter out the people who are genuinely bad at teaching. I would also hope that they don't just stick the person in a room with a camera and let them get on with it. They presumably have people on hand to help the celebrity make a good series of teaching videos.