I never heard of this hypothesis before today.  https://curiosity.com/memes/phantom-time-hypothesis-believers-think-were-in-the-18th-century-curiosity/

Why 614-911, specifically, I wonder,  The Carolingian era wiped out, just like that.  No Roland.  No Charlemagne.  No Pope Joan.  No King Offa.  No Emperor Justinian.

Why couldn't it be the years spanning the 1st through 4th Crusades?

That's a very cool theory and opens up a wealth of of historical fiction. Pity about the carbon dating and Chinese history.