If you get down to the basics, both cases use relative airflow through the blades to generate forces that rotate the blades. In the case of a helicopter, those forces also create a lift component. If a windmill generated an equivalent lift force then it wouldn't be operating at its peak efficiency.
Windmills tend to have a negative pitch, something which is rare for helicopters. So there is a substantial difference in the force diagrams.
It really comes down to how your textbook defines autorotation, and windmilling for that matter. I think most make such a special case for autorotation that the two are not the same, but I wouldn't call your instructor on it.