@Brendanjones the Why and How sections of that issue are highly instructive.
Your understanding seems correct: In the most expansive version of this vision, anyone running an AP-enabled git instance (with one or more repos) can send MRs to another instance’s repo, without having to sign up there.
For starters this will be GitLab-specific, but that’s already huge for self-hostess of GitLab who currently don’t benefit from the internal interop of the GitLab.com network.