Different PbtA games have rather different emphasis on different parts of the game. I think it makes more sense to describe it as a style with a common dice mechanic than a system.To stretch the definition some - since we're talking about adding in sourcebooks and the like, you could say that the PbtA "system" is a great universal system. The rules are the same, just the playbooks and GM agendas and such differ. It varies in that there is no core book to get first and then add a genre book onto, you just get the genre book that focuses on the experience you want to play. Again, this takes a little bit of mental gymnastics to see as a unified system so not will agree it fits the definition.