"If he was real, his story is not and cannot be an allegory. If his story is allegorical then he was by necessity non-historical."

I don't understand that at all. An allegory is the representation of abstract ideas or principles by characters, figures, or events in narrative, dramatic, or pictorial form. Who says that the characters, figures or events cannot be historical? If I was to write a story placing the main Mudcat protagonists in the Borgia courts of Renaissance Italy it would be allegorical. Does that make the Borgia courts non-historical? I think not.

If you want to use pseudo-intellectual linguistic logic please make it at least plausible.