Even if the fuel supply stopped, this does not explain the aggressive decay in rrpm - unless the response to a double engine failure was to pull maximum pitch!
Stunning that there are some who believe that "Even a fully stalled rotor should still have some rotation as long as the lever was down." As henra has said, there is a "point of no return" for every helicopter in terms of rrpm afterwhich recovery is no longer possible. You should know the minimum rrpm for the type you fly or, if you can't remember, simply do not allow your rrpm to decay beyond the bottom of the green.
In my mind I was speculating that we might have read about a catastrophic main gearbox failure but now I have no idea as to what may have happened. So many details (on the surface) don't seem to add up.