Belloc was not the originator of the critique of Islam as a Christian heresy. St. John of Damascus, who was intimately familiar with Islam, living as he did in the Caliphate (in one of those brief periods when Islam really was tolerant--although a Christian he was the equivalent of Lord Chamberlain or Prime Minister to the Caliph), wrote the first Christian critique of Islam, and regarded it as a heresy rather than a proper seperate (false) religion.
You are correct. But keep in mind that Belloc was a great historian and also a great Catholic scholar. When he wrote he wrote as such, and he quoted his sources in footnotes.