It's not guaranteed, it's limited space only. Guaranteed would not be possible because it requires 70K and only 50K can be bought. Nevertheless, for someone who is flexible, a chance of this discount is not bad. But it raises the question:

1) When do you know you can fly or not when you get a limited space ticket?
2) If you don't get to fly, does everything get refunded (miles and tax) or do you just get put on another flight?