ATSB is clear:
“The limit load for the:
- vertical load on the horizontal stabiliser was exceeded
- vertical load on the wing was reached
- bending moment on the wing was exceeded
- engine mounts were exceeded.
The ultimate load, in terms of the asymmetric moment on the horizontal stabiliser, was exceeded.
ATR’s analysis found that the maximum load on the horizontal stabiliser coincided with the maximum elevator deflection that occurred 0.125 seconds after the elevators uncoupled. At that point, the ultimate load was exceeded by about 47 per cent, and the exceedance lasted about 0.125 second.”
I suspect that the aircraft survived because the load exceedance was so short. Also surplus static strength can arise from meeting fatigue durability requirements and from aeroelastic stiffness requirements. For whatever reason, the passengers were very lucky.