Unfortunately this suffers from compounded niche-ness. Even with huge interest and constant public f-ups from social media companies, the Fediverse has failed to gain any real traction (likely due to the unintuitiveness of cross-server identity).
GitHub is already niche for the general populace, most people haven’t heard of it. Among programmers, alternative SCM platforms outside of GitLab and Perforce are niche. When we multiply the two together we get an incredibly small set of potential users (and the UX challenges of ActivityPub are still there).