Say a explorer interviews a non-literate tribe

How could we know?

By non-literate, do you mean a tribe without a written language?

 

If you read the various accounts of how the LXX came to be you will find some amazing things, almost too unbelievable to be accounted as history. I just say that because much of human history includes exaggerated accounts of tradition in their history. That is what I would expect woven among a fairly accurate account of their history if the research is done well enough. There is a reason for "two or three witnesses."

 

That sounds as if the explorer asked the tribe about the history of their mouths or their speech. I expect you meant something such as:

Say an explorer orally interviews a non-literate tribe about its history.
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But why should the "product" be a myth just because the explorer spoke the questions and the tribe members spoke their answers?

Of course, the tribe members might believe in myths, so if the explorer asked, "How did your tribe start?" the reply might be a myth, such as: "The Great Dark One who inhabits the far side of the High Mountains came here, bringing with him the eggs of a giant butterfly. He blew the eggs from his hand, and as soon as they touched the fertile soil of this land, the eggs hatched into the first people of our tribe." (Yes, I know it's corny, but it gives the idea, I hope!)

The explorer goes home, and writes up his notes for publication.

If he writes, "I met a tribe that came into being when the Great Dark One blew butterfly eggs over the land", then the "product" would be as much a myth as the tribal story.

But suppose the explorer writes, "I met a tribe that believes it originated when a mysterious being they call 'the Great Dark One' scattered the eggs of a butterfly on the land." That is not a myth; he had met such a tribe, and that tribe did have that belief about its own origin.

I get the feeling I have missed something here, and I have probably totally misunderstood your post, but at the moment I don't really see your point. Sorry.

 

Perhaps I am misunderstanding the question or thinking too simply, but it seems to me that if it is factual it is history and if not it is myth. How to verify which it is might be a problem, though.