I would say that because MDF is heavier then Plywood it is going to have a lower resonance point, meaning the MDF will store vibrations longer, much like a heat sink, and release it slowly causing a smearing of the imaging. Then by bracing in an effort to eliminate resonance mass is added, which again lowered the resonance point. Imaging ringing the bell in the tower of London as compared to the bell on a bicycle. Lots of energy off the back of the driver is headed to the cabinet.
Another illogical argument based on lack of knowelege of material resonance. Thick and highly braced MDF cabinets have a higher resonance point than thin walled designs like a Harbeth - usually in the >400Hz range. This is what happens with very stiff enclosures. The bell analogy falls apart because each would have to be struck with the same size clapper. You'd be striking the London bell with a bicycle clapper. You're basically referring to the Gandy argument of how materials store energy, problem is, he too has a misunderstanding of materials science. Put a stethoscope on any of his plinths and you'll hear rumble galore compared to a high mass design.
