The actual generator has no moving parts. The "tanks" for storing the heat can be made from graphite, but the thermal battery made by combining the storage tan

The actual generator has no moving parts. The "tanks" for storing the heat can be made from graphite, but the thermal battery made by combining the storage tank with the generator that they propose has to pump heat around using liquid tin (or perhaps liquid silicon) as the working fluid, at temperatures up to 2400C. That requires not just moving parts, but some pretty far out engineering. All of the metals we commonly build things like pumps out of are liquid at those temperatures, after all. And of course, you want pumps that run reliably for years in that hostile environment.