[Rainbow doesn't expect anypony to be surprised about her suggestion. Sickened or saddened, yes.]

Pinkie, we only got the Elements because Twilight came to Ponyville. And we only got the Elements when we used them. If we were the Element-bearers on that world, that means we had to have used them.

What I'm betting is that we ended up controlling Nightmare Moon, instead of turning her back into Princess Luna. Then we kept what happened and the name of the sixth Element a secret so nopony could take them from us or use them against us.