Iirc ground effect is going to help a lot during such low level flight, in fact there was a plane conceptualized (and trialed) to never be flown out of ground effect. That's why it has such stubby wings, the ground effect itself makes up for the absence of lift area:
That's not to belittle the capabilities of the pilots flying in these takes, it's a very well executed maneuver and certainly not entirely risk free (assurances aside, such stunts have also gone horribly wrong). But it's not quite as hard as you might think because the ground helps to 'push back' in case one of your wings gets too low. The bigger risk is probably from cross wind, which would affect the plan asymmetrically.
