I don't know about George Washington being a "flatist", but if he was, this scene in a movie might explain it.
Discussing the "Stamp Act", which led to the American war of Independance.-
Thomas Jefferson says "I would rather sink Great Britain in the Atlantic Ocean......"
George Washington (Who had been a Colonel in the Colonial MIlitia) says that "I don't know much about the ocean . My military experience was on land". (Flat Land ?)
If you are interested in this, check out the movie on the internet "Williamsburg The Story Of A Patriot", a movie which is shown at the Visitor Center in Colonial Williamsburg.