So, you've introduced a new piece of information which would have been better introduced all together with your first post. So, we will try the other CW-r rotation point.
OK, the CW-r could be one or the other rotation point. Neither of us know for sure which rotation point is to be used for the CW-r. What both rotation points have in common is that the chin point (lower jaw) will rotate backwards and down and the back of the mandible will move up with CW-r around EITHER of rotation points (ANS or insisor).
Let's try ANS as the rotation point. With that rotation point, the soft tissue of the lips and also the frontal incisors will go backwards with that rotation point.
In that case. In that case, the excess angle of inclination of the incisors and also their protrusion will be REDUCED. So, maybe he chose that rotation to reduce the excess angle and and extent of protrusion of the upper incisors.
Your midface could look flat to you via RELATIVE COMPARISON of the excess angle and protrusion of the upper incisors. It isn't flat or recessed in terms of some other angle measurements maxfax surgeons use (SNA norms).
With rotation around the ANS point (CW-r at the maxilla), the (chin point) at mandible also rotates down and backwards along with the rotation at the maxilla. However, the rotation allows for advancing the mandible along a better angle of inclination relative to a horizontal plane which makes it possible to advance it without the chin point 'sticking out too much' or looking 'over advanced'.
As is obvious from the ceph, the mandible needs to come forward to correct the bite.
What may not be obvious to you is that the surgeon is looking at a TRIANGLE made from 3 points and seeing that rotating it CW-r is the optimal rotation for your case. However, I can't draft out a long geometrical tutorial in the event the rotation of a triangle is not a concept you are familiar with.
So, all I can offer here is WHY I think your surgeon's proposal is on target with correcting your bite and also mitigating the excess 'stick out' area of the upper lip. The general concept of what he's proposing rings right (correct) to me and there is NOTHING in the information presented to opine on the extent of the mandible advancement that you say is a 'big mandible advancement'.