What you should also realize is that these blades, being relatively light-weight and connected to the crankshaft via a 1:1.69 gearbox, have to windmill a diesel engine with a compression ratio of, what, somewhere between 1:20 and 1:25, whereas a direct drive petrol engine with metal blades only has to pull the engine through a 1:11 or so compression? There is not a lot of inertia or torque in this combination to keep the engine windmilling if fuel injection halts. So I think you can safely assume that if the engine is not developing power, it will stop windmilling only seconds later, unless you really keep the speed up.
Read the AFM especially the TR's. It’s ok speculating what actually happened but until the Pilot in question actually posts on here or the AAIB publishes its findings NO ONE KNOWS!!. So lets all carry on guessing. BTW the Compression ratio is 18 –1