BS!
I think it is a twisted conspiracy theory based on misinterpretation of information presented by fallible humans.
However, a genius I knew was convinced of it. We all have our blind spots.
I suspect the "flat earth" is yours. Of course, the Earth looks flat locally. As you go high enough, you start seeing the curvature.
The curvature is easily measured with the instruments used in celestial navigation. Those instruments have been successfully used for centuries. They would not work on a flat earth.
As a surveyor, I am surprised you haven't read about the principles of celestial navigation.
As a surveyor I'm surprised you don't know this...
The London and Northwestern Railway forms a straight line 180 miles long between London and Liverpool. The railroad’s highest point, midway at Birmingham station, is only 240 feet above sea-level. If the world were actually a globe, however, curveting 8 inches per mile squared, the 180 mile stretch of rail would form an arc with the center point at Birmingham raising a full 5,400 feet above London and Liverpool. Adding the station’s actual height (240 feet) to its theoretical inclination (5,400 feet) gives 5,640 feet as the rail’s necessary height on a globe-Earth, more than a thousand feet taller than Ben Nevis, the tallest mountain in Great Britain! ”
—David Wardlaw Scott, Terra Firma (1901)
BS!