Crab,
I get the whole 'compound is just made for recirculation' angle. However, the helicopter had just flown a significant distance and therefore burned petrol. Even with a running takeoff on departure the hover performance would not have been an issue even in recirc. Unless they had refuelled enroute of course. That just leaves vortex ring. Either way as you said, if they screwed up the perf calculation that's a bit dull.