This would have had to have been an enormous conspiracy, with participants in many countries. In England, for example, this was a very important time period with the formation of the English kingdom, Alfred the Great, the beginning of English literature. That would have all have to have been made up. Ditto for the other countries. And what about the rise of Islam? Sounds crazier to believe the conspiracy theory than the standard accepted theory. But, as with any such theory, it’s going to have its adherents. I mean, look at our political world and what people accept!
A spinoff of Roman PIso Theory that had a brief existence over on the Delphi Forums was that the Roman Empire itself was faked == yes, I know, what about the roads, arenas, Hadrian's Wall == during the Middle Ages of course, and that the fake was done to create a supporting myth for the (trade) empire of the Venetians. Weird coincidence that the Venetians were mostly the adversaries of muzzies in the Seven Seas.