Suppose a mediaeval Welsh author was to give us a story - obviously false - about a meeting in Avalon between Arthur and the Lady of the Lake, and this story was unique to that source. Even if false, would its very existence represent an attestation of the historicity of Arthur? That's an interesting question, and I'm not sure of the answer. I don't think it has been discussed in these otherwise exhaustive threads either.
Well, maybe not about King Arthur, but I think I've touched on this before in a very long and boring message about applying the criteria of Bible Studies to prove a historical Cthulhu.
Well, I'm not going to do the king-size version again, but thing is, a lot of authors have written about Cthulhu or other parts of the Lovecraft universe. People have composed hymns about it. Seriously, you can get two CD's worth of hymns and carols from the H.P. Lovecraft Historical Society.
Now let's take a much smaller byte at the problem, by finding an analogy to your Arthurian example. And it will be about the Esoteric Order Of Dagon in Innsmouth, instead of Great Cthulhu, but same idea.
In the song
The Worst Hotel (sung by the Dagon Tabernacle Choir, so, you know, extra confirmation), the protagonist has a somewhat similar experience to the protagonist of The Shadow Over Innsmouth by H.P. LOvecraft, yet it's clearly a different story. He even gets an *ahem* indecent proposal, that wasn't in The Shadow Over Innsmouth. And it confirms several key points of The Shadow Over Innsmouth, without seeming to outright copy from it.
Even more significantly, we have the song
The Innsmouth Look, which tells an even more radically different story: one from the perspective of a local cultist of Dagon. It's not even a horror story, it's just a touching story of fish-boy meets fish-girl at the local EOD chapter, they fall in love, etc. But again, it confirms all the key points from The Shadow Over Innsmouth.
Hell, we now even have corroboration from two opposing points of view. The Shadow Over Innsmouth has a protagonist disgusted and terrified by the whole thing, and trying to destroy it, while The Innsmouth Look gives us the point of view of a villager who's quite content with it all. It's unlikely that if they were to make stuff up, they'd make up the same things, right?
So, to paraphrase your arthurian question... does that mean we have corroboration of a historical Innsmouth and all? Or maybe derivative fiction doesn't count for much, after all?