Objects don't need to be duplicated for forks. See the objects/info/alternates file. They're talking about redundancy for the object storage backend, which is

Objects don't need to be duplicated for forks. See the objects/info/alternates file. They're talking about redundancy for the object storage backend, which is independent of how many forks/clones/repositories. In fact, assuming they solve collision detection somehow, they could store all objects for all repositories in one object store and have all repos be thin wrappers with objects/info/alternates files forwarding to that object store repository...