In certifying an aircraft (one where Vne and Vdf are defined solely in terms of IAS) the engineers consider the performance of the airplane in determining the max TAS achievable. Freedom from flutter has to be shown throughout the flight envelope (plus a margin) - thats the speed-altitude envelope not just the V-n envelope.
As others have noted, you came come unstuck when adding more power so expanding the flight envelope with the resultant erosion of flutter margins.
One thing I like about FAR 23 airplanes is the confidence in the proof of freedom from flutter.
I know I was one of the first to drift away from spinning so, as some compensation: I also like the confidence with FAR 23 airplanes on spin recovery.