The guy's christology is all over the place, and his unqualified use of the word 'monophysite' very misleading, but he makes an interesting point regarding the possible response to tri-theism. Indeed, even if his hypothesis is wrong, what the Qur'an is attack is clearly tri-theism, either on account of misunderstanding or deliberate misrepresentation. So exposure to a tri-theist sect makes a lot of sense.
What I'd be interested to learn is whether there was some kind of Ebionite/Nazarite (or an offshoot thereof) presence in Arabia at the time of Muhammad. As you know, the Qur'an does not speak of 'Christians' but of Nasara (Nazarites). Given all the favourable mentions of Nasara in the Qur'an, it would make a lot of sense if Muhammad had had exposure to (or even based his religion on!) an Ebionite-type sect that accepted Jesus as the Messiah, but denied His divinity and emphasised selective observance of Mosaic Law.