I think McGrath is placing it a bit early by going back to Adam. The quote must have arisen after an active debate on "what is folk music" had started, which I think rules out Adam, and also Buddy Bolden.I wouldn't have thought the term "folk music" was in use in New Orleans in 1900(or in the rest of the English speaking world). A.L Lloyd in "Folk song in England" makes an important point when he refers to "Louis Armstrong's dreary axiom" (with a footnote saying maybe it wasn't Louis Armstrong's). It really doesn't matter much who said it (though it would be very interesting to know): it does matter that this dreary bit of clever-dickery