It is the case. It's not a ratchet and a spring, it's just a spring which can be compressed further than any spring before. (The phase changes along the way are interesting, but will reverse themselves as you release the pressure).
Now, if you could get a giant diamond anvil cell (with house-sized near-flawless diamonds), you could use it to compress a macroscopic quantity of this down, and then get the energy back. The problem is that while the substance in its compressed form may have more energy per cubic foot than, say, a tank of gasoline, in releasing that energy it would expand to something much larger, so your entire apparatus would be very space-inefficient for energy storage (not to mention the difficulties of obtaining those giant diamonds).
