In this episode there is a funny spoof of South Park (I think they were a lot meaner than South Park were to them in "The Simpsons Already Did It" though).
Anyway, why does Bart say there's only 43 episodes of South Park?
Apparently, most South Park episodes are done from concept to completion in less than a week, which is why so many involve issues relevant to the time of their airing. Some Simpsons episodes take up to a year to produce, especially when they involve guest stars.
My point is, perhaps this episode of the simpsons is older than we think. Is there any chance that this episode just took really long to make? Or is this episode meant to be from a previous season, like the episodes in the current season of Futurama? Or, most likely, did I miss the joke altogether?