By what I consider to be a generous definition, RLS now is generally accepted to be a "movement disorder." I say generous, because this definition includes both involuntary and voluntary movements. In the case of RLS, I do not believe that it is appropriate to include the latter.
In my opinion, RLS is the antithesis of a movement disorder. It is not an illness that causes involuntary movement of the limbs nor is it an illness that creates discomfort due to the movement of the limbs. In much the same way as a person's hand might flinch from a hot stove, I move my legs to gain relief from the discomfort/pain that I otherwise experience in the absence of movement. Yet who would suggest that the flinching of a hand following its contact with a hot stove is a movement disorder?
Indeed, I might suggest that the flinching of one's hand from a hot stove is more of a movement disorder than is RLS owing to the instinctive and hence involuntary nature of the former movement. In the case of RLS, however, compelling the need to move, ultimately, it is my decision whether or not to do so. My legs do not move on their own.
Be well.