It's not a joke at all - it's flippant for sure, but only in the face of a particular conceit that Folk Music needs a definition at all; a description maybe, but never a definition.

The Horse Definition simply states that all music is human music and all humans are folks, unless of course some folks are more folk than others, which leaves with a weird taste in the mouth. This seems to be what Steve Roud suggested a while back on this very forum (I forget exactly where) when he said that the idiomatic folk song 'Shoals off Herring' became an actual folk song when it was sung by a bona-fide traditional singer (whatever the hell that might be), thus earning it the taxonomic chufty badge of a Roud Number.

All music is NOT Folk Music (heaven forfend) but all humans, without exception and with some considerable evidence, are folks. Music is what folks do; it's what folks have been doing for 50,000 years and more (how's that for an unbroken tradition?) and as long as there are folks in the universe there will be a plethora of diverse music idioms, including Folk Music I suppose, which is only idiomatically different from other musics despite the rather silly and ultimately patronising postulations of a Folk Orthodoxy whose wayward cultural concepts go hand in hand with their reactionary imperialistic tendencies despite their leftist leanings (present company excepted, of course).

The Horse Definition sums that all up with economy and elegance, and, perhaps ironically, says pretty much exactly what the 1954 Definition says too once we've clarified some of the creaky terminology. In fact, I'll personally buy a pint for the first person here who can show me one single musical idiom on Planet Earth which can't be said to be:

...the product of a musical tradition that has been evolved through the process of oral transmission. The factors that shape the tradition are: (i) continuity which links the present with the past; (ii) variation which springs from the creative impulse of the individual or the group; (iii) selection by the community, which determines the form or forms in which the music survives....

If you really want to approach an understanding of the nature of Old Traditional English Speaking Folk Song, treat it as an ethnomusicological study in relationship to the culture, genius and virtuosity of the working-class women and men who made it and sang it in its natural habitat. Describe it, lovingly, as the cultural treasure it most surely is; but above all have respect for its feral nature that no amount of taxidermy and taxonomy can ever hope to get a measure of. Listen to the old singers with reverence and wonder; start with Grainger's 1905 recording of Joseph Taylor singing Brigg Fair and ask not What is Folk? but rather What has folk become?