I may not be totally understanding your statement. The forces acting at the resolution point (i.e. the exhaust nozzle) can't cause moments. There is not lever arm to create a moment so the forces taken at that point do not create moments.

"Summing" and "resolving" about the exhaust flange both invlove adding the component forces and moments. The descrepency I see is that to truly resolve the forces and moments, you need to add in the moments from those forces not on the resolution point. I do not believe the NEMA-23 example does this.