Interesting. If true, that would mean that, when Belloc called Islam a Christian heresy, he was exactly right.
Belloc was not the first to be of that opinion. St. John of Damascus, who knew Islam intimately, having served as Grand Vizier to the Caliph of Damascus during one of those pragmatic periods when Muslim rulers cared more about competence than Islam, wrote the first Christian critique of Islam, and treated it as a heresy.