All very well to pull the throttle back on a small piston engine but try that on a larger type such as a TSIO 520, or better yet a GTSIO 520. Closing the throttle deprives the cylinders of air and reduces compression. The piston then travels the limit of the con-rod and is smartly pulled back by the mechanical components of gudgeon pin, con-rod, crankshaft and main bearings. Huge stress!
Using the mixture leaves the throttle valve wide open and allows the inducted air (minus fuel) to at least cushion the upwards travel of the piston without loading the mechanical components.
Sure, the engine may not fail in that sortie, but may fail during a subsequent flight when an unsuspecting pilot (without the benefit of an instructor on board) and passengers are faced with a catastrophic engine failure.