> Arthur is just a name, and there's not much direct evidence the legend was a "Romano-British Chief" except via hand-wavy reasoning about themes.

Well, the earliest source we have that talks about Arthur (Historia Brittonum) is talking about a British leader. And the Battle of Badon has more/earlier sources that mention it. Was one of the leaders at the Battle of Badon actually named Arthur? We don't really know, but it does seem more plausible than the story of Atlantis.