Functionally the 2 m rule is a vertical plane.
It's not though.
Think about a disc in a tree down in a valley where the disc is below the level of the fairway (plenty of examples at DeLaveaga).
The defining aspect of the 2m is is that the disc is isn't on the playing surface. Rather, it's suspended 2m above the playing surface. If one disc is on the playing surface, and another isn't, but they are both at the same height above sea level, one is safe and the other isn't regardless of where they sit on some horizontal plane.