I am not going to argue with the AIB, they are very clever people, and I have worked with some. I will however offer an alternative for discussion.
If the uncontained engine took out some hydraulic pipes, then you could lose both systems pressure, the S76 cannot fly in manual control, but you are obviously going to try to save the aircraft. Could the control rod have pulled out under severe loading.
As I said already I am rusty now, but I seem to remember that both 1&2 systems run pipes through the tail boom to the tail rotor, if I am getting confused in my dotage, then someone please correct me.