Agreed that you can't 'discover' someplace where other civilizations already are established, but no sailors believed the earth was flat

Not only did pretty much everyone who lived in sight of the ocean know that the earth was not flat (easy enough to see as you watch a ship sail away - or watch the shore as you leave), but the diameter was also known by a number of mapmakers (diameter calculated by the Greek Eratosthenes (not to be confused with the playwright Erasthones) in Alexandria around 30-10BC using a very clever calculation of the angle of shadows between his city and a city where there was no shadow on summer solstice.

The issue is that he calculated in stadia (the length of a stadium) - and we're not sure exactly which measure of stadia he was using.

Anyway, The issue with Columbus is that the mapmaker he chose to use was using a circumference half of the more correct one in common usage. If there hadn't been an American continent in the way, his expedition would have all perished on the way to Asia - but the Americas were right about where he (wrongly!) thought Asia would be.