I think it supports both. There’s options for off grid that includes Bluetooth and multipeer connectivity, and options for multicast dns and relay servers. As for the why: it supports Android and iOS so if you don’t have an Android device, Briar is basically out.
Does Briar work on iOS?
No, only Android and any Android-derivative that can run .apk files. For more convenient installation, it's available on Google Play and F-Droid.