Honestly, I think this is just how they want to run their repository… Syncthing Fork is an independent, third-party project, so we’ve got no control over it and its management.
When it comes to GitHub, I believe a “collaborator” is someone whose PR has been merged into the repository.
“Collaborators” in this case means people explicitly added to the repository, unfortunately. I strongly disagree with this way of running things. My opinion doesn’t matter in the least, though.
No, those are called contributers in GitHub- speak. A collaborator is a GitHub account explicitly listed as collaborator in the repository settings (under the collaborator tab), i.e. for personal repos these are accounts with write access to the repository (can merge PRs, push commits etc). For organization-owned repos the permissions of a collaborator are a bit more fine-tuneable.