yea there is another top level comment by gtirloni which links to a different example which does it without a gimbal, by applying a sinusoidal current with variable amplitude and phase to the motors. They have symmetric rotors though, so your question applies there! I wonder if maybe the rotors are in fact slightly asymmetric and just look symmetrical.
edit: I got it, this is super neat: The blades are hinged at a 45 degree angle, but in opposite directions. See Figure 1 and 2 here
edit: I'm dumb, it has gimbaled props. That makes OP's link much more impressive, since that thing doesn't require gimballing which is expensive.