Perhaps they were slightly airborne, like 0.5m above the ground, but ground-based instruments recorded the plane as still being "on the ground" (i.e. same altitude as the runway) because of the downhill slope? If they really were on the ground, the pilots would have continued to feel the vibration of the wheels against the ground, a very clear indication that they aren't flying.

I'm even more surprised that the plane could land on its own wheels despite the huge gash in the fuselage. Do landing gears retract so quickly, or was the wall simply not wide enough?