As said elsewhere, it’s not that a plane can be kept in level flight with an artificial horizon that matters. As with the airbus pilots, it’s the surprise - especially when it happens in already stressful situations.

In this case we’re also dealing with an instrument SVTOL landing in terrible weather which cannot be flown with just an artificial horizon, repeated electrical failures in a short timespan, a disbelief that the aircraft correctly reverted from SVTOL to winged flight mode with all relevant instruments to confirm out - as the site says, “extremely challenging cognitive and flight conditions”. And, a manual that says if the aircraft goes out of control at low altitude, you must eject.

At a high altitude cruise it could have been flown just fine by the horizon while things get diagnosed and possibly diverted to a landing suite suitable for the remaining equipment.