The rationale here is that supply of CapEx commodities like steel and iron ore is harder to take off the market. The high fixed costs of facilities like mines makes it more expensive to suddenly shut them down, and so producers are more willing to continue producing if there's demand.
But OpEx commodities like shale oil can cheaply be removed and restarted, although that means producers have less incentive to do so.
They wrote: "For 2016, we expect the 'lower for longer' theme for commodity prices to continue, but with the additional 'demand tilt'. Namely, that China's efforts to rebalance demand from investment to consumption should reduce demand for CapEx commodities (such as steel, cement, and iron ore) much more than it reduces demand for OpEx commodities (such as energy and aluminum)."
Source: Goldman Sachs