Would have been darn hard to fly manually with the two AOA disagreeing by 20 degrees and a significant difference in airspeed left vs right. Could an experienced and well-trained crew fly it with that much disagreement in the two readings? It’s reminiscent of the Air France flight from Brazil with the frozen pitot tubes.
No not easy, but very much doable. The "standby attitude indicator" is mechanically driven.
From my reading of the preliminary report, omly the left side AoA was abnormal. But say both (left/right) are abnormal/inop flight crews revert to the standby attitude indicator. Fly it and live another day.
Its only hard if flight crews do not have a good "scan" (most do not, and it must be practiced).
Companies like Lion Air have training on using the standby attitude indicator in their training syllabus (its required for approval) but because of the "time" and subsequent cost to perform in the simulator, it gets skipped over, yet signed off by the instructor.