This would seem to be refuted by the thesis of Poor White Trash in which there was still low brow entertainment which only intensified over time into the social media era.

One of the cool things about signifyin' is how much it blurs boundaries between everyday speech/language practices and things like writing and speeches which have traditionally been the modes of communication that rhetoric scholars have concerned themselves with. High and low culture is very interconnected, which is something we see not just here but I think also in literature of the Harlem renaissance and of modernist literature (might be worth noting that the Harlem renaissance was during modernism).