My interpretation (FWIW) is that a small number of winning clients get the money, and IG 'only' keeps the spreads.
But if 70 - 80% of clients are long on a market (say), and they turn out to be right (copper, recently) then I don't see how IG could stop from losing money unless it put some kind of real trade on or fund the losses from money made elsewhere (in an underlying market where most clients were wrong)