I have always been under the impression that "Manichean" was a charge made by the church against heretic movements in the Middle Ages to discredit them. There was a serious rivalry between Christianity and Manicheism in Late Antiquity. The Christians found it particularly objectionable that the Manicheans would use elements of Christian doctrine and re-interpret them. Hence, calling someone "Manichean" was a serious offence from the Christian point of view, and if something even remotely resembled Manicheism on the surface, that was enough to cast them in the deepest pits of hell. So if the Cathars or Bogomils are called "Manichean", that is just following medieval polemics.