> The insulation keeps the heat in the ground. It protects the ground under the house from freezing.
If it is preventing the ground under the house from freezing, then it is supplying heat to that ground. That's a energy-negative process - some of the heat you generate is used to maintain the warmth of the ground.
Without under-slab insulation, the heat emanating from the ground below the house escapes very quickly. This heat we don't generate. It occurs naturally. This is about using the naturally escaping heat to keep the ground under the house from freezing, about raising the frost depth.
This technique is especially interesting for holiday homes not permanently heated in northern climates. It's fairly standard in Scandinavia, up and coming in the Baltics, but less known in the US.
In the US, it's usually called FPFS or frost protected shallow foundations. [0]
If it is preventing the ground under the house from freezing, then it is supplying heat to that ground. That's a energy-negative process - some of the heat you generate is used to maintain the warmth of the ground.