But Git is a distributed VCS. They could just as well cleanly merge pull requests into the main branch as hosted on GitHub and then merge GitHub's main branch into their canonical system's main branch.
I just randomly picked a contribution from an external party. The commit that made it into mainline has an author set to the person that contributed the code originally and a committer set to one of the 'signal.org' members. I'm confused why people would have a problem with how they chose to manage their code if the original attribution is not lost in the process. I'm fairly certain the Linux codebase is managed similarly.
Edit: adding links I forgot https://github.com/signalapp/Signal-Android/pull/10219 https://github.com/signalapp/Signal-Android/commit/dda51bf36...