This theory seems to me not only plausible
but probable
certainly much more probable
than the fairy-tale we read about
in the Koran.
1. It does not explain the contemporary account in the Armenian history that mentions Muhammad by name, specifies the monotheist doctrine, and ascribes the new political unity of the Arabs to it.
2. It does not account for the new political unity of the Arabs.
3. It does not account for the new doctrine in Syria not being e.g. Nestorian Christianity, or straight Judaism, rather something new that turns out to be Islam.
4. It does not account for the capital in the Hijaz, before moving to Damascus. It does not account for the new capital once the Omayyads take over being in Damascus, rather than say Jerusalem, or Antioch, certainly more important cities under the previous religions of Syria.
5. It does not account for the early fights recorded by the Muslims themselves being over principles of succession, not doctrine. If a new doctrine was being adopted by the newly arrived army, why was there no party of the old doctrine, and no fight over the shift?
6. It does not account for no one in the later splits within Islam appealing back to any such period or any such events, but instead all trying to claim legitimacy from the time and purported sayings of a supposedly mythical founder.
7. All of it in you most recent version is supposed to occur in the time of Omar, that is within 10 years of the death of Muhammad according to the usual history. That means, within living memory of the supposedly falsified events. Everyone in that generation would therefore have known it as a conscious fraud, which is wildly implausible. It is one thing to allege (as some of your sources do) a slow accretion process in literature over 150 years. It is something else again to allege a conscious deception taking hold within 10 years without sparking the slightest recorded notice or opposition, internal or external to Arabs.
8. The standard history (among orientalists e.g.) account fully for all of the literary influence evidence put forward (and no other evidence -is- put forward), as results of the period between Muhammad's oral teaching around 630, and the eventual Abassid codification of official texts in the 8th century.
9. The standard history's reading of the Syrian textual influence's placement simultaneously fits all of the other elements mentioned above. None of those correspondances has been argued to be wrong by the revisionist position. No evidence has been advanced for rejecting any of them. Instead, the revisionist position fights the weak straw man orthodox Muslim account, while ignoring instead of addressing the consensus of the objective historians.
10. The revisionist position accounts for absolutely nothing in the Arabic sources and their version of the history, except to reject the entire lot of it as a grand tissue of lies, motives for which, methods of which, etc, are entirely hypothetical guesses of the revisionists. No attempt to track e.g. the series of internal Muslim political disputes is made.
It explains far less, it is contradicted by contemporaneous sources, it advances nothing against the scholarly position it seeks to supplant, and instead makes up whatever it likes to put in place of the lies detected in the straw man of Muslim orthodox fairy tales. That is not objective history. It is the mirror image of tendentious apologetics.