I picked up on this too and disagree with the author. In reality, Islam is a heresy.
It began as a heresy, not as a new religion. It was not a pagan contrast with the Church; it was not an alien enemy. It was a perversion of Christian doctrine. It vitality and endurance soon gave it the appearance of a new religion, but those who were contemporary with its rise saw it for what it was_not a denial, but an adaptation and a misuse, of the Christian thing. It differed from most (not from all) heresies in this, that it did not arise within the bounds of the Christian Church. The chief heresiarch, Mohammed himself, was not, like most heresiarchs, a man of Catholic birth and doctrine to begin with. He sprang from pagans. Read More
It does not include any movements prior to that and does not include any splits from the Eastern Orthodox or Assyrians or the Syriacs or the Copts
Islam does not fit into this definition.
Islam is a syncretic religion taking aspects of pre-islamic Arabic religions (such as djinn for example), merging it with Nestorian Christianity, Syriac, Gnostic forms of Christianity and with Judaism and Zoroastrianism -- this was not a split or a protest against Christianity -- Christianity was not a major religion in Makkah or Yathrib (what is now called Medina)
If anything, the major non-pagan religion there was Judaism
Mohammad saw a chance to unite the Arabic tribes initially using Judaism as a monotheistic uniting factor (but replacing Isaac with Ishmael as the sacrifice) and then he added Gnostic pieces (which is why Jesus in the Koran says things you don't find in the Bible but which you find in the Gnostic "Gospel of Thomas" and Jesus laughs when he is on the cross -- a Gnostic concept of Jesus not as a real man of blood and flesh -- Lady Heron, if you mean Gnostics instead of Marcionites I would agree -- because Marcion rejected the OT, while the Moslems don't.
I will state again as I've said before -- anybody who acknowledges Christ as Lord, God and Savior is on the same side as me against Moslems -- I even support Zoroastrians, Hindus, Sikhs and Buddhists against Moslems, even more so I support fellow Christians against Moslems
Christians have been fools twice before - when the Moslems invaded Egypt, the Copts supported them as they thought this would end their persecution at the hands of the Catholic-Orthodox. This was a mistake. Ditto when Orthodox lands were invaded -- Catholic lands put up paltry support until the Crusades
And finally, at the siege of Vienna, Lutheran Hungarians fought on the side of the Moslem Turks against Catholic Germans and Hungarians and Poles -- but the Lutherans are not the only fools, 200 years later Catholic France and Anglican England fought on the side of the Turks against Christian Russia preventing the retake of Constantinople to Christianity
Fellow freepers -- do not let history repeat itself. ANY Christian attacked by a Moslem is an attack on you