jj as effectively used is a git UI. There's an experimental self-hosted backend that nobody uses (and is disabled in release builds), and there's a piper backend in Google, which is enough that people call it a VCS of its own rather than a git UI, but there's nothing stopping some other tool building the same primitives on top of git directly without jj as an intermediate layer. As I understand it, to some extent, that is GitButler.