not quite. Basically, think of the top layer being a slab of steel. Then a cut out in that slab, or island that isn't coupled to the steel slab. The motor hangs down on this island. Under that is a 1.25" thick piece of soap stone. This implies that there needs to be a cutout in the soap stone for the motor to hang down, and various other cutouts for the idler and the arm. Then the motor island is bolted down to the soapstone, as is the entire steel plate. Then, there is a constrained layer damping layer of.... IKEA cups? eventually, birch ply. Perhaps a few layers of EAR C-1002 ISODAMP thrown in for good measure, then another 1.25" thick piece of soap stone. All in all, around.. what 150 lbs? heh.
The AC motor is controlled via a Mark Kelly kit in fact, an improved "to-be-commercialized" version of what you linked to. It's three phase quadrature drive.. Ironically, I've sorta halted things as I need to shape up the soapstone plinths a bit, as I've gotten the arm board worked out and everything. Then I'll need to complete the birch ply layer.... lots of work, so little time.