Another big, lingering question is why Meta wants to do this in the first place. Lambert says Meta wants to give users more control over their posts and followers, with easier avenues to engage across platforms.
So will they be implementing a method to export this data in ways that could be imported to other platforms? Otherwise I don't see where federation fits in here all that much.
Extending reach isn't really the same as control imo.
...Would this have been local posts of an individual, or sometimes a group in a LAN or something? The way you describe it here puts me in the mind of recent stuff like Scuttlebutt, albeit that's more clearly individual-focused.