Is Ibn Warraq’s claim credible that Meccan–Medinan differences reflect divergent sources and origins, not Muhammad?

He suggests that the Quran is a collection of different sources both older and younger than just Muhammad (eg. old Syriac Christian homilies, Umayyad-Abbasid creations) which explains its heterogeneity. The sira and Hadith about Muhammad were fabricated later to try explain this. Is this idea credible today?

*“There is indeed a difference in language, style and even message between the so-called Meccan and Medina suras…The differences, if anything, point to a history far more extensive than the short life of Muhammad… there is nothing natural about the Meccan-Medinan separation. it is clear from Lammens, Becker, and others, that large parts of the sira and Hadith were invented to account for the difficulties amd obscurities encountered in the Quran, and these labels also point to be convenient for the Muslim exegeted for the Sam reason. The theory of abrogation also gets the exegeted out of similar difficulties and obviates the need to explain the embarrassing contradictions that abound in the Quran”.* (The Hidden Origins of Islam: New Research into its Early History, pp.246-7).