"I guess we can rule out friendly Muslim adventurers"
Since there was no such thing as Islam prior to the 7th century, that's a good bet, but that "twelfth emperor" thing would probably sound rather Muslim, to a Muslim.
My comment related to the expanded discussion showing commonality between Aztec, Mayan and Incan beliefs. Since the last documented Mayan architecture was completed around 823AD, and the Incas died out in the 1400's, surely exposure to Muslim adventurers is possible... though unlikely?