I think you've misunderstood. The system you're describing kicks in when (a) the aircraft is on the ground, (b) commanded thrust is idle and (c) the engines continue to produce thrust above idle (when there is an overthrust condition). Boeing does not cut power to the engines to prevent reverse thrust application in air - that would be absurd.

On this particular occasion the system is thought to have malfunctioned in some way (e.g. it might have not registered the application of reverse thrust) triggering a shutdown.