The app cannot function as you'd expect a screen saver to do: it won't start running after a period of inactivity. That is outside the scope of functionality available to apps.
Hm. But you can still use it for the purpose screen savers were originally designed to do: save screens from getting burn ins. But agreed: it can't run automatically after a period of inactivity.
Off topic: As a child I loved to look at screen savers that I manually started for hours wondering how they work and how long it took to build them...
