The market making exemption of the Volcker Rule is based on RENTD - Reasonably Expected Near Term Demand. So Goldman has to prove that this was the case for the high yield bonds. Proving this could be quite tricky though - because they have to factor in a whole lot of parameters (market maturity, depth, liquidity, product holding periods, macro-economic outlook and the trading desk's estimation of client demand). But I'm sure Goldman has models to track each of these. So all in all, this is quite a windfall