You are right. Ptolemy taught of the rotating spheres. So, the concept of flat earth somehow coexisted with the roundness of the world as a whole. Indeed, as one observes the sky with the naked eye, he sees the visible universe round, but the earth flat.
The spheres were assumed to be "in heaven" and anything in heaven could only be perfect, which is what spheres are.
Potlemy's real contribution was to affirm the old World Order by creating a set of mathematical formulae that predicted the orbit of any of the observed celestial bodies at any time and gave birth to modern navigation.
The beauty of Ptolemy's success is that is ti based on a false premise, and earth being flat was not it.
Ptolemy's system was based on the model that has the earth at the center, and everything else (including the Sun) in orbit around us.
This scientific model only reinforced Aristotle's "explanation' why things fall on earth, i.e. gravity ("because things fall towards the center") and the Church teaching that man and earth are God's central creation.
Here we have science, philosophy and religion converging and reaching the same (independent) conclusion. This is how we nowadays come to "truth." The convincing power of three most important fields of knowledge of that time established an order of truth that was almost unshakable, even when presented with clear-cut evidence to the contrary.
I was wondering if any of the "educated" in the west ever forgot the round world deal. The problem the round worlders had with Columbus was not that he'd fall off the edge but that they knew the Indies were way further away than he said they were.