The code included in the GitHub repository downloads the closed source Electron binaries. Just for comparison, the full source would look something more like what's accessible here: https://github.com/electron/electron
Thanks! I see it as unlikely that they will release the code, because castlabs also seem to have a business of auditing implementations for consideration for the widevine program: https://github.com/castlabs/electron-releases/wiki/EVS#3pl