That’s much too early for Arthur. I’m not buying this theory.
Arthur’s tales from the Middle Ages (which were cooked up into their present form by the French) were based on very much older tales; they are part of a broader tradition of tribal, pre-Roman Celtic Britain. My favorite conventional post-Roman Arthur hypothesis (can’t remember the name of the author, the book is, I think, “King Arthur: the True Story”) puts the ‘real’ guy in a formerly Roman town in western Britain. Ah, here it is:
http://www.grahamphillips.net/books/arthur.htm
? That’s weird, page won’t load.