So if I'm understanding right, this isn't like LineageOS[0] for Android, which builds a system from the Android Open Source Project (AOSP)[1], but a "romhack", or modified system? Pity. I naïvely hope that running a fully open source system would be easier on a phone like this, since it's simpler than the latest and greatest in the smartphone world.
Realistically, they probably still run the same radios which never have sources available.
[0] https://lineageos.org/ [1] https://source.android.com/