Yes Dave, you are right, hydraulic lines run all along the tail boom to supply the tail rotor servos, but if these lines are severed the "tail rotor shut off valve" will automatically isolate the main rotor stage 1 servos from the leak, as the hydraulic level drops. Thus, in case of a major dual system hydraulic leak aft of the the main rotor servos, control of the main rotor is normally still possible via hydraulic stage1.
....I'm not "speculating" but as a S76 flyer, it seems normal to share ideas and experiences on such a case.
For sure, investigators will soon find out what happened.