So the investigators are saying the test pilots caused the crash by doing something they shouldn't, and understandably that is being received with scepticism on here. But is it possible the feathering deployment was a test to make sure the interlocks/software etc would prevent it?
Take a completely different situation, try to raise the landing gear while you are on the ground and interlocks/pressure switches will prevent it - but at some point someone has tried to do it, to make sure it doesn't happen.
The only way to test these potentially dangerous side effects is to test them and make sure the systems prevent you doing it - and if they don't then you are in trouble.