That was the point I was making; the only way it would work would be if the controller was on-die with the eDRAM and communicated with the CPUs over Infinity Fabric, in which case it'd hardly be any better than going to main memory. A proper controller wouldn't add too much die space, but obviously it's something that would have to be added in Zen's successors.
(Of course, in an ideal world AMD would fill the space with a nice big HBM2 cache, but even if the necessary controller logic was there I shudder to think how much the required interposer would cost)