Where are you getting the idea that Egypt ceased to be a net exporter of grain in the Middle Ages? The stranger phenomenon is the the fact that what was Africa Province (essentially Tunisia) became a net importer of grain. Africa province was a major agricultural center during late antiquity, and that status diminished in the Middle Ages. Venice, in particular, imported African grain, as Chapin Lane has shown. Tossed Rapoport and Ido Shahar have demonstrated that the Mamluks actually improved irrigation in Fayyum in their article “Irrigstion in the Medieval Islamic Fayyum.” Now, you don’t have a trunk route of grain taxes from Egypt to Constantinople, supporting an “artificially large” civilization, but this does not discount the fact that Egypt was a major grain producer during premodernity. Furthermore, it is worth considering that the ENTIRE eastern Mediterranean was not supported by Egypt during late antiquity, but rather Constantinople and maybe some very large cities. Much of grain consumption would have been from local production.