He was taught Orthodox Christianity by a Monk, his concubine Mariam was a Coptic and he traveled in the lands of both the Orthodox and the Copts.
Belloc is wonderful - actually, when I was young and reading my way through people of that generation, I didn’t like him very much. But it’s amazing how the years bring wisdom...
Yes, Mohammed definitely knew about Christianity, including its orthodox form. Personally, I think he’s a lot more like Joseph Smith (or vice versa), because he wanted to create a form of vaguely Christian-based religion that reflected his time and place. Just as Joseph Smith placed the Garden of Eden in Missouri, so did Mohammed relocate and redefine the holy places, persons and concepts of his religious environment to serve his Arab supremacy cult.