Let me give you a run down of the board state.
My opponent had Forgestoker Dragon on the battlefield and a lot of other, smaller creatures. He could pay to kill any other creature that came out on the battlefield, but had no effects in his deck to deal with Elderscale Wurm nor Canopy Cover.
Similarly I had no way to deal with his dragon, and if I attacked with my Elderscale Wurm it would die, and I would lose to his horde.
Aether Rift says:
At the beginning of your upkeep, discard a card at random. If you discard a creature card this way, return it from your graveyard to the battlefield unless any player pays 5 life.
Library of Leng says:
You have no maximum hand size. If an effect causes you to discard a card, discard it, but you may put it on top of your library instead of into your graveyard.
My opponent only had creatures in his hand. He wanted to use the interaction between these two cards to randomly discard a card, not pay 5 life,(I could not pay the 5 life cost because it would bring me below 7 life, negating Elderscale Wurm's effect) and have it go back to the top of his library, so that he would have 1 more card than I did, thus I would mill out before he did.
Is that how this interaction works?