"Proprietary" means having owner(s). If the maintainers are the only ones allowed to introduce changes, they are establishing an ownership relationship over the codebase. I.e., it's a proprietary project, and that is the sense in which it's closed.
This is the destiny of most projects that go AI, in my opinion.
https://exquisite.social/users/thomholwerda/statuses/116697078838020135
https://ladybird.org/posts/changing-how-we-develop-ladybird/