Dropping the sphinx means you don't have a farm there, which means it's just a plain ol' tile in terms of food and production. You won't get plenty of pops from a floodplain until it has a few floods, which may take some time if it ever even happens.So? You'll have plenty of population thanks to those floodplains and trade routes.
Cleo's trade route ability grants the food to other civ's using her as a destination, not her. And it seems to not really attract other civ's particularly well.