They aren't actually interacting with MTGox at all. They are just using the MTGox price as a reference for settlement of CFD
(Contract-for-Difference) contracts that they are offering.
So how do they hedge against their users leveraging in the opposite direction that they are on?
If a user shorts bitcoins on their platform are bitcoins actually borrowed or sold? This is what I'm unsure about.
I think with CFD you only allow trades between two users. The house never takes a position. When the contract ends or a margin call occurs the loser's margin is transferred to the winner.