Sand is generally the last option to choose for thermal mass, because it is so inefficient at storing heat; the very air between the grains acts as insulation.
Instead, clay (or clay mixed with straw to form cob) is the preferred medium for storing heat, as it is a significantly better conductor and stores much more heat energy than the air between the grains of sand.
It seems like they are using the air as a medium here- the article states that they blow air through the sand, and the resistive bands heat up the air, which then sheds some of the heat onto the sand (or picks some heat up off the sand, depending on if it is storing or discharging).
I guess the cheapness and simplicity of construction makes this a better option at the scale they're operating at, but it's pretty wild to imagine how much more efficient the system could be if it had been designed differently.