I figure that he's thinking of ancient Britain, ancient Jews, and Atlantans all as mythic stories, where the culture rises, blooms, and ultimately falls, either from cataclysm or invasion.
A good book to view the Jewish culture is Isaac Asimov's "Guide to the Bible," which recounts many rises and falls of that culture, wherein the power of that culture is always exaggerated. It never was as strong in the region as the Bible says it was, and was routinely gobsmacked by other cultures. It had its mythic heroes and tales of great deeds, but very little of it was historically verifiable.
Similarly, we're never sure whether Arthur really existed. He was probably not a High King, but he might have been a warlord of that era. He might even have been Vortigern, as some historians have suggested. He might have been a legendary Celtic superhero. All e know is he is the center of a mythic tale that describes the rise and fall of a man.