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.