What is one reason historians might believe di Coppo di Stefani Buonaiuti’s account of the Black Death?

One reason why this is believable is because di Coppo Di Stefano Buonaiuti was in Florence at the time when the Black Death hit Florence in 1338. One reason why this passage isn't believable is because he was only two when it happened so might not remember it all.

the conditions that made the byzantine empire vulnerable to attacks by the muslin turks were the sack of constantinople by western crusaders, the conquests of the sultanate of rum, presence of rival kingdoms and finally the set up of fiefdoms at the empire which were owned by the ghazis (turkish warriors).