In other words, Manchuria, or what is now Northeast China, was once Korean, from the top on down? Wouldn’t surprise me...
Would you say that, fine, if Goguryeo and Baekje were Buyeo speaking people whose legacies and traditions got absorbed into a non-Buyeo (I think Mahan) Silla, then wouldn't Goguryeo and Baekje be the great ancestors of the Japanese?
It's a really touchy topic I'd say.
By the way, can you translate what it says in the history of Japan chronicles. I'm guessing that's in classical Chinese?