It's not clear how exactly they ended up profiting on this. It says one of the playlist "had 467 songs by virtually unknown artists," like the artists used were inconsequential.
Did the Bulgarians actually have the rights to all 467 of those songs in order to be the ones collecting the royalties? Did they cut deals with whoever did own the rights?
It sounds like they recorded 467 songs of their own and published them on spotify. I.e., they're virtually unknown, as in unknown to anyone other than the people making the playlist.