In fairness to Tony Cunnane, in his account of the incident he states that he "saw the mach meter hovering just over mach 1". You may well be right about inaccuracies in calibration, but all Tony could go on was the evidence of the instrument. What does surprise me is they had been cruising at .90. In general we used to cruise at .84 - maybe they were just in a hurry to get home after the detachment!
Btw I have a shrewd idea I know who the co-pilot was, and if I am right I am not in the least surprised!