Because that's not all it is.
They didn't just take a console and slap a solar panel on top and wired that into the battery compartment.
They took a new CPU, buttons, etc. and run an inefficient emulator (goes to show how big the energy budget is) with auto save state before turning off from lack of light/button energy.
Arguably, the former is way more useful in the short run, as you could actually use it for more than ten seconds at a time and retrofit any device easily. But as a research project, the latter is way more useful because it might pave the path to this being viable in the future and having an entirely new class of devices.