You could argue it's more like a precursor. Proust was at the same time probably trying to achieve a narrative form of Bergson's "duration" as time-mixing in consciousness (he and Bergson were best friends). And Joyce, it's theorized, got hold of an early copy of Merleau-Ponty's PoP when he was developing Ulysses. So the sub-genre or whatever you want to call it didn't come about in a vacuum and serves different purposes for each author. Then again, Modernism is even less of a real genre than SoC is.