Well, the relationship of flutter to IAS and TAS is certainly a
puzzle to me.

Somewhere I got the understanding that IAS, in a sense, indicates the
impact rate (pressure) of molecules on the aircraft, and thus in
thinner air an aircraft will "actually" (TAS) be flying faster to
receive the same air molecule impact rate (pressure).

TAS, on the other hand, indicates, in a sense, indicates the speed
at which the air molecules are moving past the aircraft - something
quite independent of just how MANY air molecules are passing by
the aircraft in a given amount of time.

Further, I have had the impression that flutter is a consequence of
the speed of the aircraft through the air (molecules) (TAS) rather
than the number of air molecules that happen to be impacting the
aircraft in a given amount of time (IAS).

So, I have always considered it prudent to view VNE due to FLUTTER
to be a TAS airspeed, not an IAS airspeed.

Have I been wrong about this?