Yeah washout has nothing to do with it.

Flutter is equivalent to when you boost yourself on a park swing to go
higher. Your body is the aileron, and the swing chains are the wing
structure, and you move the CG of your body in a harmonic relationship
to the swing to excite and then amplify its movements. The aileron is
doing the same thing if its CG is aft of its hinge line.

By balancing the aileron and putting its CG on the hinge line the
aileron is incapable of adding to the energy in the vibrating wing
structure so stiffness is less critical. Its like laying out horizontal
on the park swing so if someone moves the swing your body just goes
along and can't amplify its motion.

John