I’m surprised that Jim and David didn’t independently invent flarf on their own, like Leibniz and Newton, since they both use humor and oddness in their writings. In fact, I’m not totally sure they haven’t, since I don’t have large amounts of their work available to read.
For me flarf is primarily about absurdity in the service of elation and subversion. That’s the upper limit. Lower limit is making your friends laugh during their tedious day-job work hours.