The biggest flaw in that argument is that flutter is not uncommanded movement of the whole model but, as you initially wrote but then ignored, just in a control surface. Gaps in the hinge line allow the air to flow in an unintended fashion between top and bottom of the airfoil. This disturbance in the clean flow can easily cause flutter. Flutter of one control can very often not affect the overall control of the model too much (until it gets so bad that the surface parts company).

Servo stalling or blowback are not flutter and I've never heard anyone suggest that they are. However your analysis of other things that can go wrong but which are not flutter seems reasonable to me, if a bit irrelevant to a discussion of the effect of gap sealing on flutter. BTW gap sealing will not improve stalling or blowback either.

Steve