Zecora is the best possible person to be spreading the story of Nightmare Moon. Please turn your head to the moral of Zecora's opening episode "Don't judge a book by it's cover"
Why is it that no one in town recognizes Twilight Sparkle's costume of a wizard from ancient history, but Luna gets it straight off, right down to the details? Luna's spent a thousand years on the moon: She doesn't have the intervening history to muddle her memory of the past. (Compare to someone in stasis easily recognizing someone from their own era, even as they fail at pop culture.)
Why is Princess Luna suddenly speaking in Flowery Elizabethan English even though she never did in her first appearance? Nightmare Moon never used it because she wasn't really Luna, but her personality warped by evil. She never used the "Traditional Royal Canterlot Voice"; her voice and speech were completely different from Luna's, like everything else about her. When Luna returns, her only spoken line is to her elder sister Celestia—no need for "thee"s and "thou"s, which another troper pointed out are used to refer to equals or lessers, not big sisters.