To be technical, it's more that it can read and write the on-disk Git format directly, like many other tools can.

I think the easiest way to conceptualize it is to think of Git and jj as being broken down into three broad "layers": data storage, algorithms, user interface. Jujutsu uses the same data storage format as Git -- but each of them have their own algorithms and user interface built atop that storage.

Yes, jj is its own VCS with pluggable backends.

Google uses it with Piper, their centralized VCS.

Being compatible and being purely a frontend aren’t the same thing.

No, that's just a practical approach to supporting migration to jj.

If we want to improve systems, we need to be able to migrate conveniently from older systems to better ones.

You can still use git worktrees in a colocated repository. jj workspaces are a different, but similar capability that provide some extra features, at the cost of some others.

git is the storage layer. JJ's commands do not have to, and never did, match git one-to-one.