There are indeed no flat surfaces on a sphere, but if the sphere is really big then any small chunk of the surface would look pretty darn flat.
you keep saying that. maybe you should be working on coming up with something that's actually true.
Here's something that's actually true: If you take a circular area 1 mile in diameter, the difference at the center between a flat plane and the surface of an 8,000-mile diameter sphere will be two inches. Are you going to notice that, even in the absence of
any local topography on land or ripples on water?
"Pretty darn flat" describes it well.