Slow heat transfer (and the absolute need to minimize capex) drives the system to operate on very long (seasonal) time scales.

... and similarly doesn't it mean the pile is slow to absorb heat when your PV installation is trying to dump energy into it?

You don't need to store it in the dirt. You use the dirt as insulation, and store in something like molten salt or whatever which can be pumped up to surface and has good thermal conductivity to then extract the heat. At least that's my understanding of all these systems.

Read the article.

He is talking about storing the heat in the dirt and he gives good economic reasons for that.

The point of the article is that storing in the dirt is much simpler and cheaper than molten salt.

Deliberate exchange of heat would be done with internal radiators designed to maximize the transfer.

Environmental exchange would be limited to the interface between the storage tank and the surrounding soil.

It should be orders of magninitude more efficient to transfer energy intentionally than what would be lost to the environment.

It means the heat stays near the hot pipes for quite a while.