Do people read this unironically? I am at the beginning and already is factually wrong. European population continuously grew from the 7th to the 14th century. And the rest seems to have been written by a 19th century 'historian' or someone who only read their books.

I've read a bit more. It's worse. Charlemagne was the grandson of Charles Martel, who famously stopped the largest Umayyad razzia in Europe (and used it to take power, and consolidate both Neustria and Austrasia). His empire was never threatened/invaded by the Umayyad, even after his death. He created the Gascony march, that did threaten the Vascon and the Umayyad before Louis le pieux was defeated by the Vascons, who created the Navarre realm, basically creating a buffer state between the Umayyad and the Carolingian empire. So for sure, they had no impact on wether the Carolingians could maintain power or not. Also, the reason Martel could take power was that the Franks already had a feudal society since at least Clovis. Also, the Carolingians did maintain power, everywhere. The successions divided the realms, but they kept power for a long, long time (I still count Capetian as Carolingian for obvious reasons)

And I'm really not a specialist of this era, I find the 16th-19th century more interesting overall. Basing any arguments on that shaky knowledge is uninteresting to me, so I will stop there, his point might be good, but the argument to reach it will be bad.