MacColl was not averse to passing his own songs off somewhat trad, eg "from the singing of Jimmie Miller " and who was this Jimmie Miller?

And that is in the public domain too.

If his songs are not Folk Songs - tell me how could he collect one from a lumberjack in Canada who swore it was from the singing of his Grandmother. And at the time the song was maybe less than 20 years old. He wrote in a style that was not that distant from Folk Song as we know it and those songs live in that very space now. Good enough for me.