I think it boils down to the definition of "rapid" throttle movement.
Obviously a snapped close throttle is detrimental to the good health of the engine. I do not see a measured throttle closure of (say) 3-4 seconds as "Rapid". Have you ever watched the auto-throttles go from idle to full power on a CFM 56? Now I call that pretty fast throttle opening by what I was used to on other engines - but that rate of throttle movement is normal, otherwise the engine manufacturer would not design the autothrottle system that way if it was going to compromise the engine operation.
What has that to do with a piston engine throttle movement you may ask?
The answer lies in the perception of the operator (pilot). A steady measured closure of the throttle should be perfectly acceptable engine handling for a simulated failure. If not, it is doubtful that the engine would be certified. After all aero-engines go through unbelievably rigorous testing during the certification process.
It also drawing a very long bow to accuse a testing officer of stuffing an engine in the future by his decision to close the throttle rather than the mixture. Did the engine tear down positively confirm this as the reason for the engine to fall apart?
What was his rate of throttle closure? Was this rate of closure conveyed to the engine manufacturer and their opinion recorded?
It is all too easy to blame the last pilot for a perceived wrong action when there could be other factors present beyond his control.
Interestingly some years ago in Australia the chief pilot of a C402 operation was killed along with six haplees pax when his aircraft suffered an apparent engine failure shortly after lift off. He failed to get around to feathering the dead prop and soon after flipped inverted due Vmca.
It transpired that he and all his pilots had only ever carried out their proficiency checks - which included simulated engine failures after take off - in a PA44. The engine failure procedures and associated handling were entirely different. It was all to save costs of course - but proved in the end to be false economy...