We'll be saying more about this very soon, but the short version is this: we are not building a GitHub competitor in the way that SourceHut or Codeberg are. We are building infrastructure for enterprises, not a social coding site.
While jj works with git, it doesn't have to. jj has the ability to work with many backends. Git is the popular one in the open source world, but that doesn't mean that other ones are not possible. And that's the core tech we have built.
With the rise of agents, more and more companies are running into the limits of what Git can do. It was never designed for large monorepos, and so there are performance issues, but also features that it's missing, that are important once you start trying to scale things up to an entire organization. Companies like Meta and Google ran into these things well before the present moment, but you don't have to be a FAANG these days to be running into this sort of stuff.
We also understand that incremental adoption is really important, especially for something as fundamental as source control. So our tech also has a git ingress/egress that will work with the core functionality, but more advanced things that git simply can't do will end up requiring using a non-git protocol. jj is the bridge between those two worlds.
I hope that makes sense! We have been in psudeo-stealth for a while now, this is the start of us coming out of our shell a bit and actually talking about what we're doing.