absolutely right. a point that you are missing here is that my theory contends that many stone circles (henges) were used for grain processing. see section 5 of my thesis.
http://www.granaryatstonehenge.org
this part of the theory is probably more important than stonehnege being the only mechanical mill.
i purpose that stonehenge (and others) were grain working areas from the very earliest age. it only became a mechanical mill in its last phase.

thnaks, clyde