"fully coherent single address space with all memory available to [all processors]" is a lot easier if there is no virtual memory, no caching, and all processors are on a single shared bus to the same memory chips.
> Single-socket nodes will consist of one CPU and four GPUs, connected by AMD’s custom high bandwidth, low latency coherent Infinity fabric.
The article, at least, only describes coherency in the context of single-socket nodes. I suppose it's ambiguous whether Infinity fabric only connects components on the node or connects nodes to each other, but in fact Infinity fabric is the interconnect used with Zen CPUs, so I think it's pretty clear that in this case it only connects and provides memory coherency for 1xCPU and 4xGPUs.
Performant memory coherency across nodes would be quite an achievement, but not one they seem to have achieved. Rather, it seems they've simply revived the PS4 model.
I feel these things always go back and forth in cycles in the industry.