The app is so relatively new that initially it wasn’t even on any of the app stores, only straight from their site. But now it’s also on Google Play and APKPure, possibly other alternative app stores as well. Give it some time, I’m pretty sure F-Droid is on their radar as well sooner or later.
The app is still labeled as being Under Construction, but so far I’m rather pleased with it and would recommend giving it a try.