Constantinople alone managed to hang on as a civilized state while the Mohammedans in TYPICAL Muslim style bankrolled their “empire” by looting, plundering, slave dealing and warfare with one another and their Christian neighbors.
Islam is a piratical faith and a piratical culture.
This book is just so much pablum for politically correct imbeciles. If the Muslims had not turned the Mediterranean into no man's land for western Christians they could jolly well have gotten all the Roman and Greek Culture they needed from the Byzantines, without having it distilled through Islamic hands.
The Dark Ages of Western Europe began with the collapse of the Western Roman Empire around 400 AD. (Note, however, that the Eastern Roman Empire, based in Constantinople, survived just fine well into the Middle Ages). Islam was not founded until the 7th Century.
There were lots of problems with barbarians, and the Islamic contribution definitely kept things bad for Western Europe for more centuries than would otherwise have been the case.
An interesting aspect to the fall of Rome was that it happened during a Global Cooling period, when dropping temperatures produced lower harvests (and thus less revenue to support the Legions), plus causing mass migrations of barbarian people looking for better farmland.