Christianity is too broad a category to exclude them. This seems to be an American born obsession. Europe had established religions, and they disregarded any "other" sects, aka cults (of sectarian Protestantism). But this idea came to America which has no establishment of a religion, so defining who the establishment religion is, and the cult is, outsiders to the establishment is incoherent. But the 80's Americans were just obsessed with using/misusing the word cult, after Branch Davidians and Heaven's Gate.
Identifying them as Heretic or Apostate Christians is at best the only coherent things to say. Even ancient Christian heretics were identified as Christians, we still call them Gnostic Christians, Coptic Christians, Jewish-Christian groups that were (Judaizers).