Yes, I saw that interview before but he did not say anything positively or negatively on how the material of sides/back affects the timbre (not the loudness, that's obvious). He talks about importance of the wood strength and hard lining so the sound waves get reflected back to the soundboard. But still some amount of sound penetrates and travels throught the sides and back (again, depending on the hardness of the material). Also, half of the radiated sound energy travels from the soundboard through the air to the backboard and gets reflected back where it interferes with the soundboard again. Now, the backboard is another drum, even though made of much harder wood, and has it's own freq reponce and resonant modes, so the the reflected sound is shaped in frequency domain by the frequency-dependent impedance of the backboard.
Sorry, again, I'm not a luthier, but I'm a physicist, so I find it unlikely that the material of the back/sides does not affect the sound. But I would be interested to know if it is true or not based on the real-life data. And most interesting - how the type of the back/sides wood affect the sound and what are the sound characteristics of different types of wood? And is it true that the BRW produces the best sound?