Cool, interesting design!
I still think that even though there is no physical friction, this will rob you of a few watts. Aerodynamic drag will rob you of the most power, but the harvesting of kinetic energy using magnets will still exert an effect on the wheel – you cannot generate energy with magnets without the magnets also exerting an equal slowing effect upon the spinning aluminum rim.
https://en.wikipedia.org/wiki/Conservation_of_energy
The bigger problem with this product, though, is that it'll get stolen off your bike super quick when parked. Not that other lights are much safer, but if you have an integrated light that's drawing from your main battery, it's harder to steal since it's wired in (granted, that cable can be cut very easily).