Anyway, horses do sing, and so do elephants.

Though the quote gets overused and misused, I take it as a way of rejecting the kind of picking and choosing that Cecil Sharp practiced when he decided only some of the repertoire of his singers counted.

It's really a way of saying "I am going to sing the songs I want to sing, and when I sing them they are the same kind of songs wherever they came from." It shouldn't be taken as a way of pretending there aren't distinctions to be made, but rather of denying that these distinctions should be used as limits mechanically imposed on what we do.