I thought all the eggheads kept saying the King Arthur stories are apocryphal and that he probably never existed, and this story appears to be saying just the opposite.
I think that most scholars think he is a composite of several petty kings and warlords over 2 or 3 centuries
Was there a coherent Romano-British polity that mounted a reasonably united effort, at least in the early stages, or was there a swift collapse into extreme localism, with sub-Roman provincial nobility quickly devolving into local warlords? We don't know.
Was there a singular leader who distinguished himself enough in this period that he stood out to contemporaries as the hero of the piece? We don't know that either.
The pro-Arthur camp argues that the emergence during the dark ages of the folkloric tradition of an heroic resistance led by a great war leader suggests that there was such an outstanding figure. The derivation of the name is a separable question.
Oceans of ink have been spilled speculating about Ambrosius and his possible connections with, and possible identity with, Arthur. Arthur's name crops up later, in fragments of Welsh sagas and in Nennius. Arthur is thus apparently an early figure, but the name can't be definitively pinned back to fifth century.