But maybe MacColl's father composed the tune.

MacColl's lyrics are rather different - and I think superior - to other versions, including Percy's, which is the one familiar in poetry anthologies. The improvement seems to suggest a sophisticated reviser.

However...the tune is still in the folk tradition, i. e., sounds exactly like a Scottish folk tune usu. sounds (more so, probably, than the modern tunes for "The Great Silkie" and "The Twa Corbies"). The same goes for the lyrics. Whoever revised them knew what he was doing, but that's not necessarily a sin. And MacColl did learn a number of ballads from his parents - which makes him a traditional singer, at least when he sang those particular songs.

So MacColl's possible messing with tradition is still within the tradition. Otherwise its significance is for us pedants only.