You are confusing the concepts of insulation and reservoirs.

When 5 of the 6 semiaxes of possible heat flow have no temperature gradient, the temperature becomes much more stable.

Insulation would be if a 10 ft radius dirt ball maintained a stable temperature all year round - which would surprise me, although dirt does have some insulative value. However, wet dirt is really not very insulative - try sleeping on the ground sometime.

The thermal time constant of a 3d object scales as the square of the linear dimensions of the object, so time constants measured in years are quite achievable.

The dirt here will not be wet in most of its volume -- the initial charging will bake out all volatiles. The hottest part of the pile will become something like dry brick. It's like storing energy in ziggurats.