In a normal helicopter, the helicopter is exerting a force on the rotors and thus feels an equal and opposite force that needs to be countered, either by a tail rotor or a counter-rotating rotor. This helicopter doesn't do that. If anything I'd think it might need a tail rotor to stop it from spinning in the same direction as the rotor, but I guess friction between the helicopter body and the rotor is low enough that this isn't a serious concern.

(The linked youtube video shows a version of this helicopter that does have such a tail rotor, though positioned closer to the helicopter than it might normally be.)