If they're talking about production capacity, that is some product of die area and process steps, right? It doesn't have to be 3x die area, just 3x lower factory throughput for the same number of functioning memory bits.

HBM is less dense at a water scale than DDR because of all the vias, but each memory but is as dense or denser.

You make HBM instead of DDR and the number of bits you're making goes down.

It's really that simple.