We know this because it has so many different versions, points of origin, and people claiming to have had a part in its creation. Any time you have a song that simply refuses to be pinned down, rest assured that it can truly be categorized as “folk.” If there is a known author, then the most you can say is that the piece is “in the style of” a folk song. I have been fascinated to read the Wikipedia article on this piece; the various claims and counterclaims are so multi-branching that they almost form a spider’s web.