The difference between ground-speed and airspeed is the wind. In normal conditions, ground-speed is a useful proxy for airspeed - perhaps not good enough for an airliner precision approach, but more than adequate for normal flight.

For the Lion flight, winds were light and the aircraft was at low altitude, so ground-speed would have been very close to airspeed.

In any case (as pointed out by earlier comments) a proficient pilot should have no trouble flying in daylight and good conditions using only the "Mark I eyeball". It appears the aircraft crashed because the crew didn't fly it - a surprisingly common occurrence for automation-dependent crews!