He's not flying, he hasn't gone high enough to stop benefiting from the pushback from the ground. See also "Ground effect vehicle", "wing in ground effect", "ekranoplan"

Look out in this sort of video for the lack of sustained level flight at a height substantially over the width of the device/craft (1) if they can't stay at that level, it's not quite flying.

1) https://en.wikipedia.org/wiki/Ground_effect_(aerodynamics)

"When an aircraft flies at a ground level approximately at or below the length of the aircraft's wingspan or helicopter's rotor diameter ... an often noticeable ground effect. "