Geography in medieval Europe was rudimentary. In this period Cairo was known in the West as Babylon. China was thought to be at the source of the Nile, as its silk was shipped down that river to Babylon/Cairo.
The Western name derives from classical usage: the Roman fortress, next to which al-Kahirah was founded, was called Babylon-on-the-Nile.
Cf. T/O map
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.