Then explain why an oscillating pressure wave on the control surface won't impart an oscillating moment on the surface
It won't if the surface is constrained with sufficient strength to resist the oscillating forces.
And furthermore, if the oscillating forces were being degraded by the disruptive effects of blowthrough at the hingeline, and that disruptive effect is removed, the oscillating force might, just might then be able to flutter the surfaces. But probably wouldn't on model airplanes unless the rigging to that surface was sloppy.