This is also a misconception. The Invention of Science by David Wootton has a long chapter on this. They believed it was impossible because in aristotelian cosm

This is also a misconception. The Invention of Science by David Wootton has a long chapter on this. They believed it was impossible because in aristotelian cosmology the planet earth is actually two spheres: a sphere of earth floating in a 10x times larger sphere of water, so that only a small dome emerges from it (both of them are then inside a larger sphere of air which is inside a sphere of fire). In this model reaching china by sailing west is impossible because the sphere of water is absolutely enormous and all the possible emerged land is already known so there can not be any antipodal land you could find to restock on food.