Yes, the depth buffer and the color buffer would be part of the same render back end. In GCN terms a RBE has 4 ROPs. So you take the four depicted in the diagram and multiply that by 4 and you'd get 16.
Frankly it's a bit weird for MS documentation to break them out individually like that, but it might explain aegies' confusion about color and depth compression being some kind of "special sauce", but the fact is that's a pretty old idea. the old Radeon 9700 had like 6:1 lossless color compression and 32:1 Z-compression in hardware. GCN rates are better than that, but it's not new or remarkable, and it's nothing Orbis won't have as well.
Damn, MS must have an ace in the hole...