What sort of backups are you after? For a system-wide backup, I don't bother. I have the git config for the system in a remote repo. Rebuilding the system is a matter of bootstrapping and then just pointing nix at the config. For my data that is outside the scope of nix, i just have a rclone cron that syncs to/from a remote store.

Given the flake for freshrss doesn't have any sort of explicit "backup" option, my hunch is the author is doing something similar. Point the data dir to a location that gets automatically sync'd elsewhere.