The Second Opinion

At least, if that's not what they're flat out saying, it seems pretty heavily implied, with Nightmare Rarity going on about everything she remembers from being Nightmare Moon and saying that Rarity is "gone."

so possession of the highest degree? why Rarity?! why not Twilight!

Exactly. Granted, they try to give some justification about how Rarity was the easiest of the elements to corrupt, but not only does it still seem underwhelming, that also means the princess of the night turning into Nightmare Moon was purely a coincidence.