We might ask why the certification process involving the equipment manufacturers (IRS and EFIS), the airframe and simulator manufacturer, and various national regulatory authorities involved in certification did not identify this hazard.
Here we have two different hazards, and by design there's logic that gives priority to one of the hazards (unusual attitude) vs. the other (instrument miscompare). There are a number of risk-based assumptions in creating that priority list, which now will need to be revisited.