If I recall, Sprocket used charades to teach Doc about the word "Fraggles". I'm not too sure, though. And Matt does kinda blend in with all the other weird creatures sitting all around. The Fraggles don't really seem to acknowledge Doc either, even though at least Gobo should be surprised to see him there, even if it's before he learns Doc's name. Only Matt looks up at Doc and Sprocket, but they're all just in a happy singing mood, so it's hard to tell if there's any recognition.