Made a mistake there, was supposed to be Milan, not Genoa. It's Zen 3 + CDNA.
I skimmed through the article and missed the date as well, it says mid 2021 deployment so yeah definitely not Zen 4. It's not necessarily Milan though, could be the "custom" CPU they are designing for Frontier.
Edit: From the Anandtech article - "Forrest clarified that this custom CPU is not in the same way that Intel defines custom – i.e. it’s not simply the same silicon with adjustments in core counts / frequency / cache. The CPU for Frontier will be a fully custom design, built with CPU-to-GPU IF links in mind, without any excess. When asked if IF is going to be a connectivity in other platforms, Forrest would only confirm that it’s the connectivity for Frontier. Though, for what it's worth, AMD announced back at the Frontier unveil that the CPU would ultimately become available for other enterprise customers as well."