Agreed, the approach I played around with was interesting and fun getting it working but it certainly seems to be a bad approach, single source of truth as you mentioned being one. Having a nostr github replacement would definitely still want to store repositories in a traditional git repository rather than nostr events 🙂
Discussion
Is that code available anywhere? I was thinking about tinkering with this and I feel like it would work just fine, and yeah pretty easy to just read and write the history from a channel or thread. The issue is the collaboration. It would feel different. It would have to be all fully remote forks, each identity, but that still works good. full PRs for everyone, everytime.
git-nostr will be adding content (git patches base64) to nostr messages. Not up for debate because single source of truth on a publicly known git repo is a target for censorship.  https://github.com/colealbon/git-nostr.
Good luck