Taken from the report
He pressed the button to activate the fire suppression system
This would suggest the aircraft was fitted with a fire system.
The pilot subsequently heard a second “bang,” and was unable to control the
helicopter.
Reading this one does assume these two events were together and the bang was the tail boom failing. It could have of course been the squib firing (he would have heard it in flight, I know this) had the engine gone below 50%N1 and the boom failing simultaneously or very shortly after.
Of course by this time either way no fire bottle could have made a difference as the fire had apparently spread outside off the engine bay.