This a hoot.

First Folk prescribes its own sources then continues to prescribe its own limits.

The bottom line has to be that Folk is that which aspires to represent that which it considers Traditional in terms of the limits of its own inner aesthetical / ideolological sense of musical righteousness.

Those old songs, ballads, shanties & rants were never FOLK in the first instance, but only called FOLK by folkies - a particular academic / social / cultural hobbysist elite operating at several demographic / historical & cultural removes from The Tradition they claim to revere (i.e. none of the present shanty crew have ever been hardened ship-board hardened tars who sang this stuff before mast in the 19th century).

Once we've established and accepted that then - yes, indeed, It Is Really Folk Music.