At Boeing, I worked on proving the elevators would not flutter. It wasn't about a driving force being at the resonant frequency. It was about the interaction of spring rate of the system and the force pushing on it.

The same thing happens if you stretch a rubber band in front of your lips and blow on it. Increasing the stretch will increase the frequency.

Musical wind instruments work the same way.

P.S. the elevators did not flutter in flight test or in service. Phew!