On one hand the construction was fascinating. On the other I was surprised they only (?) got 30dB reduction out of a drum kit right outside the room.
I'm no expert in absolute dB levels of sound but, once they got out of the garage, the iPhone SPL app registered 50dB which, on my Radio Shack meter, is what I get in my room at night with no fans, no cars and no breathing! 100dB is way louder than I'm comfortable with in my medium-small apartment living room. There's no way neighbors are going to hear him playing (at least much) from within their dwellings so long as the app he's using is remotely accurate and they have their doors and windows closed.
We don't cover sound isolation much here but I thought this was a worthy share.
Rod Gervais' book Home recording studio, Build it like the pros, is a great resource for this kind of thing. It looks like the ceiling is not de-coupled because they "sistered" the lower (inner) joist to the upper (outer) joist hence coupling the inner to outer ceiling even though the walls are not directly coupled. Maybe that's why they only got -30dB? I wonder why they sistered the joists? Maybe they calculated they needed the extra strength to carry the load? But seems if apart, each would have only half the load? Did they isolate the walls from the floor? I did not notice. Also, seems they did not isolate the floor itself, did they?
For inside a house/garage to inside your neighbor's house -30dB to -50dB may be enough but from Studio to Control Room you want as much as you can get. It's especially difficult and important at the low end. It's relatively easy to silence vocal range but quite another matter to eliminate the thump of bass or kick drum.