This may be true, but if it's going to be used for housing foundations regardless (because there aren't competitive alternatives), then it's essentially a sunk cost. The fact that you can get a supercapacitor out of your foundation for very low incremental cost is amazing (assuming they can scale the technology as anticipated).

One thing I wondered about is what happens as the concrete expands/contracts due to weather, or is fractured when it 'settles' (or if there is an earthquake).