To the ancients, the world did indeed seem flat. You look around you with the unaided eye, and the horizon circles around you on a straight line. This gives the impression we live on a flat disk. And the stars circle around us, giving the appearance that the gods made the universe literally revolve around us. A reasonable idea for a world inhabited by many kinds of gods and superstitions. No wonder we have such big egos. :-)

But they never tested this idea. There was no science as we now know it. The ancients thinking the world was flat could be called conjecture. It was very religious too. Creator gods and all that. You can't even call it a hypothesis because it was never tested.