No it isn't. Many people and scientists did believe the earth was flat in the middle ages.
In order for your statement to be true, no intelligent person in the middle ages would have believed in the earth being flat.
You are welcome to try again.
This goes back to the old 1st grade Columbus story. You know the one where Columbus thought the Earth was round, and the monarchs of Europe thought he was crazy. The truth was that everybody believed the Earth was round, but ironically Columbus was wrong in believing it was smaller than it was. Columbus was rejected at first because learned people thought his route to Asia was too long. They were right. What neither side knew was that there was a land mass in between, except for fisherman who kept the secret of their great spots across the Atlantic.
As for intelligent people in the Middle Ages. They had access to the writings of the ancient Greeks, since the ancient Greeks believed the Earth to be round, then they had the knowledge. I will concede that same as today, some intelligent people in Middle Ages believed in some flaky theories.