As I said, there are many things that contribute to flutter. And you're right, proper linkages and servos are the first step. But not the only step. And I never said to rely only on sealing the surfaces to reduce flutter. I said to reduce all contributing factors as much as possible.

Air leakage through a control surface gap changes the pressure differential between the two sides of the surface. Sometimes this happens in an oscillatory fashion. When this oscillation sets up, it can lead to flutter if the control system is not strong enough to resist the forces. So the point is that while a strong control system and stiff surfaces are important in keeping a pressure oscillation from turning into flutter, doing anything you can to help eliminate the possibility of aerodynamic oscillation is also a good idea.

Sealing a hinge gap can create flutter? I don't buy it.

Honestly, the most important reason to seal gaps is improved aircraft performance. But if you don't care to waste your time on such things, that's cool by me.

Dave