*Conjecture warning*
IF the throttle is jammed open, presumably in a piston engine helo the Nr is now dangerously high in an attempt to stay S and L - yes?
So he flies slow or hovers to soak up the Nr, or switches the engine off - yes?
It seems there was a discussion along these lines with ATC/FI.
So he apparently opts for an EOL (not an auto as some state - an auto would drive the Nr off the clock and damage the blades - because he can't control revs). So he enters an EOL state and revs are now totally under his control.
Whatever happened next - 2/3rds of the way down the descent is anyone's guess, but like CRAB says - he must have reduced the Nr below that which is recoverable and the rest is history.