Mike Harding used to sing My Brother Sylvest in folk clubs. Makes it a folk song by at least one definition.

Bob Dylan songs that charted are pop songs on Radio 2 in the day and folk songs on Radio 2 on a Wednesday evening.

Songs written by Richard Thompson (usually genred pop or rock on iTunes and Amazon) appear on websites of traditional Irish songs, ditto many MacColl songs.

No such thing as pop. No such thing as folk. It's all music. Jim Carroll likes it to get old men warbling out of tune pretending they learned it at their mother's knee and many American mudcatters think you had to sit cross legged with a flower in your hair forty eight years ago to call it folk.