Sounds like a good solution to the problem of Photos library exceeding available disk space on Mac. Will keep an eye on this and wait for proof that it works as claimed and that privacy is secure.
The app doesn't require an account or login and the backups are stored only on your device or chosen drives and not on any external servers.
I need to add a privacy policy to the web page to make the policy clear and I believe Apple also requires a privacy policy before the app can be officially released.