That's not true. You're trying to be correct where it doesn't realistically matter, and you're not correct.
I've seen 2 american bass HD 12's flex the grill of a car and that's not even connected to the interior. Deadener stops rattling by lowering Fs of the metal by adding mass, adding mass to make the panel require more energy to move the same distance, therefore reducing flex from pressure.
Every single piece of metal on this car will vibrate with those subs LOL, probably everything with a 50 foot radius of this car is going to vibrate, if it's all correct.
Whether or not the box directly pressurizes the trunk is irrelevant, there will still be pressure differentials around the trunk panels, if not on the inside, then on the outside. Have you NOT EVER heard bass outside of car? That's why you'd deaden metal like he did, even if it's sealed off. It's still going to vibrate like crazy. Bass is still going to flex the trunk....there's bass on the outside of the car too lol. People driving next to him in traffic should probably deaden their cars too.