The stable nature of the spinning rotor seems to indicate that the unsymmetrical mass at the gearbox-end of the mast IS NOT SPINNING
Recall that the rotor head teeters, so it is possible that while still rotating as a whole, the mast teetered a little relative to the rotor assembly, so that it all came back into balance again. As the top deck pitch links will have all broken, the swashplate was likely still torsionally fixed to the mast, so both blades could go to their low pitch stop, which would aid their autorotation. The mast and transmission would definately stabilize the whole assembly in the feather axis of both blades, so the whole thing could have been very stable, and ideal for autorotation. The rotor assembly + mast + transmission appears in the video to be autorotating nicely.