As a member of an NBA team this guy presumably flies fairly often. If a plane reaches an altitude of 35k feet or higher and you have a clear wide view of the horizon the curvature of the earth can be seen. The ancient Greeks simply by observing ships on the horizon came to the conclusion that the world was curved.
The horizon is curved because the edge of a circle is always curved. When you fly, you just get a higher view of the curved edges of the circular earth.
The airlines are in on the conspiracy also. The windows are radius-ed to cause the curve of the horizon.
lol