Ewan MaColl was a person who sang both folksongs and lots of other songs, including many he wrote. Calling him 'folk' doesn't make all the songs he sang folksongs. The same goes for Pete Seegar and Oscar Brand, who rarely sang a straight folk song. They usually revised it enough to make it a 'Pete Seegar song' or an 'Oscar Brand Song'. That process of revising folk songs and calling the new version 'folk' is a continuing process.
The subject has been discussed in various threads here for years, from long before I got acquainted with the Mudcat Forum.