Well their instance is open though, it's just that the official Gitlab instance has more features that aren't released in the OSS repository of the Gitlab software.

Still, if/when forge federation happens, I think it would be amazing if all major organizations that self-host their forge used one that supports this new feature (Forgejo), so the need to make all those sign-ups, usually just to open a single issue (shudder), on a million websites would finally vanish.
Who knows, if the idea picks up, eventually we might see it implemented into Gitlab too!