That’s what I was saying at first too, but I think he’s right. Look at Skyhouse Raleigh for example.
For residential with concrete framing, mechanical is often encased in a bulkhead to one side of the apartment, so all you have is the thickness of the concrete slab. There’s no continuous plenum. The concrete slab is probably no more than 7-8" on top of 9-10’ ceilings.