There are three separate attitude (artificial horizon) indicators in the B737. Seeing the natural horizon would not be required to recover from an unusual attitude. Even if one attitude indicator was inoperative, the first thing pilots are trained to do is to compare and see which attitude is wrong, and if the supporting instruments (altitude, airspeed, vertical speed, heading) support the conclusion.
And manual control was possible. The problem was with the automation.
This is well known. My point is that if there were outside horizon and total manual control avail/in use, the aircraft should have been recoverable.