ATSB were absolutely confounded as to how the left engine appeared to have suffered damage at impact consistent with it operating at stabilised idle. The conclusion was that the engine 'auto-ignited' after all water was exhausted by the windmilling engine.
The engine was not operating is the only inference to be drawn from the report (tests made on the exhaust stacks). Plenty of water and another contaminant was found through the FCU and system. I don't know where you get the idea it was running at idle.