'It would also appear to exclude the performance-oriented arrangements of people like Carthy and Seeger. '

In that case, it really is nonsense. I can't see why something copywrighted and published can;t be a folksong. It just means the creator has a slim chance of getting paid for his work.

What folk do with the song when they have taken possession of it - that's what makes it folk music - in my view.

What are the advantages in excluding people and saying - your music is not folk music?