IFixPlanes, thank you, and indeed I am aware of the fact that there is a standby gyro - perhaps even one that I am currently familiar with that provides not only attitude, but also altitude, Mach, Airspeed and even glide slope information.
As stated in the report, the No2 2 (co-pilot side) primary attitude indications were disabled, and if I have understood your previous post correctly, the No1 side attitude indications would have taken 30 seconds to stabilise. This being the case, the ONLY source of reliable attitude reference would be from the independent standby gyro.
I have observed many situations in the sim of crew following an unreliable flight director / primary attitude indications for quite some time before realising their error only after the aircraft (sim) had gotten well out of shape. In the case of this aircraft upset, only 46 seconds elapsed before it was too late, and initially, as stated in the report, the required corrections to bank angle were not excessive.
Only my humble opinion from observations in the report, but as stated previously I am not familiar with the 734, so stand to be corrected.