It shouldn't come as a surprise to anyone that this needs a special metallic surface to work on. The basic dynamics of how gravity works means that a generic "anti gravity engine" is impossible, or at least so far from modern physics that it's barely worth thinking about. Long physics lesson short, normally compression of the ground beneath you is what counters the downward force of gravity, causing you to stay put since the forces are balanced, and forcing a board to provide that counter force without help from the earth isn't easy. The fact they managed to force "just magnets" to do so in such a controllable and efficient way (given the board stays at the same altitude from the ground it could use a lot less energy than expected) should impress anyone who's tried to force magnets together as a kid and realised how unstable they are.