IMHO the requirement that the song has been adopted into and modified by the community is correct, but the requirement that the song be anonymous is probably no longer correct

It is the genre as a whole we're looking at here; the collective cultural process by which said genre lives & breathes & perpetuates itself by virtue of its own transfiguration. The essentially occult & objective nature of this process gives rise to the subjective brilliance which is it immediate and singular manifestation, very often upping the anti for the entire genre, restructuring the whole thing. In jazz, think John Coltrane, who was only building on what others had done before him; think Rahsaan Roland Kirk who took it onto another level. In folk, think Davie Stewart: an idiosyncratic genius giving voice to a tradition; he may not be actually writing it, but in purely corporeal & creative terms, he is the medium conducting a very essential seance. The individual voices of rap are similarly mediumistic; the tradition being reinforced in fierce competition with clear rules wherein skills are honed and perfected and the music as a collective entity is enforced and sustained. Whether this makes it folk music or not is another matter, because folk music, in any case, doesn't actually exist, but it does make it a powerful and beautiful music and a force, and manifestation, of human musical genius on a par with any other.