Git makes it easy to make copies, but its nature of having one designated "origin" remote per repository that serves as upstream source of truth (on platforms like github also across forks) makes it not really federated/decentralized the way Mastodon et al. are.
And I don't reaaally see how this tool solves that problem. Or any of the other problems you mentioned.
But that said, Mastodon/ActivityPub don't really feel like mature solutions yet. It's incredibly opaque how instances interact, and self-hosting is an exercise in frustration as you try to figure out why A sees your reactions but B does not and you can get notifications for C's reactions but not A's, but B's replies and not… I gave up after a while. It's more opaque than even email self-hosting, at least exim and postfix have fairly verbose error logging.
And I don't reaaally see how this tool solves that problem. Or any of the other problems you mentioned.
But that said, Mastodon/ActivityPub don't really feel like mature solutions yet. It's incredibly opaque how instances interact, and self-hosting is an exercise in frustration as you try to figure out why A sees your reactions but B does not and you can get notifications for C's reactions but not A's, but B's replies and not… I gave up after a while. It's more opaque than even email self-hosting, at least exim and postfix have fairly verbose error logging.