Maybe, maybe not.
There is a firm tradition among Arab Christians that Mohammed started as a Christian missionary to the pagan Arabs, sent by the Assyrian Church of the East, who went rogue and set up as a warlord. This tradition received support from textual analyses of the "received" text of the Qu'ran used by modern Muslims that show passages of it, which are nonsense in Arabic and thus the subject of all manner of fanciful interpretation by Muslim jurists, are prefectly sensible East Syriac, the language of worship of the Church of the East.
I have observed that the Assyrian bishop of what is now Kirkuk at the time Mohammed began his preaching was one Mar Gabriel, and that there is an ancient custom in the Christian East of calling the local bishop the "angel" of his church (attested in the title of the Orthodox Bishop of Bosra and Haran, "the Angel of Haran" and in the Apocalypse of St. John, addressed to the angels of the churches of Asia Minor). If my surmise is correct, then before he went rogue, when Mohammed said he had received a message from the angel Gabriel, he was just saying he'd gotten this from his bishop, not asserting that the Bodiless Power who had visited the Virgin Mary at the Annunciation was talking to him.
The passages may not have been plagiarized, just passed on per instructions, then incorporated into the whole body of the text most of which was generated after Mohammed set up on his own to justify his predations.
Fascinating, do you have sources for this or is it mostly your personal observation? I would love to see your points delved into further.