RVDT,
This incident occurred in the UK at the end of November at just above sea level. (5 degrees, QNH 1025 DA -1520')
Let me assure you that if anyone in the UK today (my local DA -56') were to start up a 135, the ground idle selection would see the Nr & N2 settle at 75%. I can also assure you that if that they were then to move the twist grips to the red button position, the Nr & N2 would decrease and settle at 60%. On repositioning the twist grips into their detent'd and then selecting flight, the Nr & N2 would stabilise at 97%.
No assumptions, fact!
DB, I would suggest that the pilot took the action of fully closing the twist grips in order to prevent the engines running post impact, thereby saving a lot more lives.