>> "'you can't find one author so everyone/ the people wrote it'"
Who said this - did I miss something? <<

He might be thinking of F J Child's half-baked ideas about communal composition, which owed something to theories propounded by the Brothers Grimm in connection with folk tales.

However as far as I'm aware those ideas were discarded more than a century ago, and did not inform either of the main folk revivals. It wasn't the same concept as song evolution via oral process.