I suspect the confusion might lie in the terminology. If the students meant forward airspeed as being air that flows through the rotor from the forward motion of the disk, pushed or pulled, through the air (forward motion relative to the airflow this would include vertical descents) then I think they are right. Considering that the comments are in context of autorotation and helicopters I suspect they were referring to airspeed as the air flow over the blades, which in a helicopter can happen in two ways 1) the driven rotor pulls the air through or 2) the rotor moves through the air forwards relative to the airflow (autorotation). A helicopter can fly with no foward airspeed (hover) and the driven rotor provides the airspeed over the airfoil to create lift, without a driven rotor a helicopter can only fly if the rotor is moved forward (relative) through the air. So I think in that context you do need forward airspeed (relative to the airflow) to autorotate otherwise there is no air passing through the rotors. Also I think the term forward airspeed is just a way of referencing the direction of the relative to airflow so if there were no visual clues and the pilot was in an open cockpit he would have the wind in his face and it would feel to him (or her) that the aircraft is moving foward (even in a vertical descent the pilot would feel the wind coming from the front). You can be flying backwards relative to the ground but you are still flying forwards relative to the airflow.