I can only think of one way for a room's contribution to frequency response to be a function of volume: absorption.

If my sound treatments completely absorb 200Hz @ 85dB, but not so 200Hz @ 100dB, then you could compensate for the additional reflective gain when at 100dB. Your cut filter would only be active then.

But it seems what people are describing on AVS Forum is that because our hearing is less sensitive at high frequencies, and likewise people like feeling bass, then if you are listening at -30dB Audyssey Dynamic EQ will boost bass and high frequencies so you can still hear/feel them. Subjective observations.