Linda is probably the single-most prolific poster to Arthurnet. And as such, she's also probably the single-most disagreed with poster to Arthur-net. her claims are pretty solid and defensible, but as with anyone's claims in Arthuriana, they're hardly unassailable or incontrovertible. The more I read arguments on Arthurian issues, the more skeptical I become.
There are many sources for Arthurian stories, and few stories actually about Arthur (who would have to have been a 5th or early 6th century figure) exist before centuries after that.
That's like somone today starting to tell stories about a pre-colonial figure. A figure who was world-famous but that not a single word was written about till today.
I'm sorry, I'm a huge fan of the myths and legend, but the man wasn't real.
As for being based on someone, it's fairly hard to swallow that he was based on "one man," esp considering how scarce records were, even among the Romans, but esp there on the fringes.
Incidentally, there are also some very strong theories that Arthur was Scythian or Russian. However, it seems to me that the most logical position is that Arthur was Welsh or Britonic (sub-Roman British) and that he was either based in Roman culture or rebelled against it. The actual history is that the (ethnic) Romans had pulled out of the land -- a land they had never really tamed, but had left behinda number of institutions and architecture; much of which was either adapted or fell out of use.