The author also ignores the behavior of the First Crusade, which beseiged Constantinople on Holy Thursday of 1097. Anna Comnena, daughter of the Emperor at the time, writes of the Crusaders that they travelled in the guise of pilgrims to Jersualem, but had the intent of seizing the Imperial Throne.
You are correct, the crusades and the venetians laid waste to the Byzantines, and laid the groundwork for the Muslims expansion.