There are distributed forges? Yes, git is distributed, but often everything around it isn't. The case parent is trying to make, is that the rest ("federated forges") should also be distributed, not just git.
> what is the missing killer feature that needs federation?
Issues, pull requests, collaboration/permissions/access, "staring"/"favoriting", etc.
I think ultimately the goal is that people can run their own forges, yet still collaborate on repositories hosted in other forges, leveraging your existing authentication so you no longer need to sign up individually for each forge.
I'd say we have emails, mailing lists and bug trackers. Or maybe: what is the missing killer feature that needs federation?