Well, if Doc had only been TOLD about the Fraggles by Sprocket (how, I don't know, since that dog can't talk to make out the word Fraggle), it would mean he'd never actually seen them. If he had, wouldn't he say something like, "Are these anything like the Fraggles back home?". And I'm not sure, I think the Fraggles only appear behind him for that one song, but he never turns around.
Maybe they didn't want the two things to mix like that, though. I don't know, I'm all confused.