I like that!
I think one point is that during the summer, the underground tubes cool the greenhouse air, and that is part of the heat storage. That is, the summer heat charges their heat battery so that winter energy is higher than would be available just from the ambient soil heat.
Also, note that snow can actually be a good insulator. And even in winter, there can be a lot of clear days with a lot of sunlight. Snow throws a lot of that back, but a greenhouse can capture that heat.
A good greenhouse could also keep moisture inside, so even dry areas might be viable. I am thinking of the crappy desert parts of southern Wyoming.
