Was listening to his books while walking the dog when I came across this passage.
But in the wearing of the swift years of Middle-earth the line of Meneldil son of Anárion failed, and the Tree withered, and the blood of the Númenóreans became mingled with that of lesser men.
He was a scholar of ancient Aryan lore, folk tales, and histories. That's what he's based his fantasies on.
There are warnings of race-mixing from the Oer-Linda book and from many sources long millennia before Christianity. We have been fighting this fight forever.