NVA is not alone in supporting this approach - for example DNM have been using 100% acrylic cases (except for the heatsink on the power amp where the transistors are spaced from it by ceramic blocks) for years.

Electromagnetics also back up the theory - cables will produce an electromagnetic field from them, and metallic objects nearby can pick up the nearfield, the electrons in the metal can start oscillating (like a passive antenna element) and re-radiate back to the cable.

Best,

Andrew