I think you might be misunderstanding the scope of an "input". It's something like "the player pressed the key/button that maps to move forward", not "move to position X". Teleport hacks wouldn't be possible in the former case because the player has to actually move their avatar to the desired position by traversing the game world through a series of inputs. The server would then process the movement inputs, and the avatar would say, bump into a wall, preventing it's movement. There would be no input for "my position is now X".