I'm pretty sure his understanding of myth differs from yours not least in the respect of the literary and historical expertise he had.
Well perhaps we should use the dictionary definition - indeed the Oxford dictionary definition which, as an Oxford academic, I guess he would acknowledge:
Myth
1 a traditional story, especially one concerning the early history of a people or explaining a natural or social phenomenon, and typically involving supernatural beings or events: ancient Celtic myths | [ mass noun ] : the heroes of Greek myth.
2 a widely held but false belief or idea: the belief that evening primrose oil helps to cure eczema is a myth, according to dermatologists.
• a fictitious or imaginary person or thing. nobody had ever heard of Simon's mysterious friend—Anna said he was a myth.
• an exaggerated or idealized conception of a person or thing: the book is a scholarly study of the Churchill myth.
So, although 1 doesn't quite iterate is specifically there is a clear indication that the story isn't actually true. @ goes further to make clear it is a 'false belief' - therefore the notion that a false belief somehow becomes fact is a non-sense statement.