Certainly a very solid construction.
Though they are playing a bit with words: They write that the walls are "massive" while a little further down on the page they describe that they have "milled grooves" inside the wall!
They are absolutely right that the construction becomes thermally inert. But not as inert as a stone house.
Now, I have not scrutinized their calculations, but I am a bit doubtful that the insulation becomes quite as good as they claim with the help of their measurements - statistics is a very plastic medium.