I didn't forget, I just lumped those under "mass" for simplicity because that's what they are, masses. Sure they work more effectively through the mix of materials but like I said, they can engineer a soundproof room in a room if they want but that's beyond the scope of a simple comment. Two layers of wood like you have in both examples is a not inconsequential amount of mass.

Also need to be careful with any design because the triple leaf effect is real and weird and doing layers like that inside a room might not have the desired effect, especially if they overlook coupling to the floor/ceiling and just created a drum instead of a dampener.