Not an aeronautical engineer but I can see a couple reason why the propulsion would be on the retreating side of the rotor. When a helicopter is hovering the air speed on both blades is equal causing both blade to be balanced in lift. when in forward flight the air speed of the retreating rotor would be less. Since the blades are fixed there is no cyclic to cause forward motion and the collective is a fixed angle.
to fly forward on this machine it needs thrust from the jet engine. being on the retreating side the suction of the intake air combined with the out flow of the thrust in my opinion increase the air flow on the retreating blade allowing it to have a closer resemblance to the air speed of the advancing blade. But this seems counter intuitive since lift is caused by higher pressure under the blade with a lower pressure above it due to the curvature and attack angle I would think the suction of the intake while it would speed up the air flow it would also create lower density in the air stream under the blade disrupting the laminar flow creating turbulence, Perhaps the thrust of the engine being many times greater than the velocity of the intake would be supplemental to this.
But as I said I am no aeronautical engineer