As it happens, I have a copy of King Arthur: The True Story, so I did a quick browse. And the problem is, the earliest mention of this individual comes in the tenth-century Annales Cambriae. By this time, the historical record has already been corrupted by the Arthurian legend, so in the end I don't think the similarities in the names prove anything. I'll add Owain Ddantgwyn, if he existed, to the list of people who might have contributed to the Arthur legend. But he is no more the 'true' Arthur than any of the other candidates.
And I had to sit through Piers Morgan blathering about Cherie Blair.