Your right. It doesn't modify the game. But if you load the seed in a single player world, you can get the coordinates of where all the diamonds are. Because the world is the same always for the same seed (and same version), you can just go into that server, go to the exact same coordinates, and the diamonds would be found there. So pretty much it is cheating, if not, worse than x-ray.
Could you send some images/videos to show us this is happening? If you could also tell a method that shows how to reproduce the issue it would be even better.
Your right. It doesn't modify the game. But if you load the seed in a single player world, you can get the coordinates of where all the diamonds are. Because the world is the same always for the same seed (and same version), you can just go into that server, go to the exact same coordinates, and the diamonds would be found there. So pretty much it is cheating, if not, worse than x-ray.