First I've heard of this, but they seem to be a young, smart and energetic team on a great trajectory. The atproto choice is interesting, fits their social-minded goal.

How easy is it to set up and run this for private repos?

It looks like it has so many cool features -- stacked PRs, jujutsu support, CI in Nix VMs. But I've never tried it because it sounds like (at least by default) it's some sort of decentralized, peer-to-peer public thing