Speculation Time! I wonder if Nightmare Moon is the Anti-Luna in the same way this episode showed us the Anti-Pinkie, Anti-Applejack, Anti-Rarity, Anti-Fluttershy, and Anti-Rainbow Dash.
I also notice that Twilight was the only pony who didn't get inverto-zapped. How would you show "not-magicness," anyway? By having her only engage in mundane and unimpressive behavior? With her horn gone anyway, I imagine it would be hard to tell the difference.
-The ( SPOILER )
PS: Discord totally cheated. Make a rule, then coerce (via mind-control) someone into breaking it so you can say you've won? In law enforcement, that's called "entrapment!" But then, he's Discord, did anybody expect him NOT to cheat?
PPS: Just for jamesbarrett: Pinkie does Maramelldansen!