The point wasn't really about having your own copy of the commit history, it's about having your own copy of the refs (which is all a branch is in git). Basically, your master branch is not the same branch as GitHub's master branch or anyone else's. This is one of the things people don't really seem to understand about git. It means you don't have to do the "feature branch" thing, for example, you can just do commits on your master branch then submit a PR.
Yes, branches are just mutable pointers. The commit history is an immutable tree and has a garbage collector to clean up.
Funny enough, this is more or less exactly the architecture some of those Haskell-weirdos would come up with. It's essentially a copy-on-write filesystem.
(Haskell people are weirdos compared to good old fashioned operating system people who use C as God intended.)
