They use the classic AGPL + license assignment CLA combo.
This lets them relicense community contributions however they want, including making them proprietary. Everyone has to abide by the AGPL terms, apart from them. This puts them in a privileged position compared to the rest of the community, which I don't like.
Edit: @artyom@piefed.social pointed to a commit that removes the CLA from many places in the repo. They didn't (forgot to?) remove it from the LICENSING.md file, which is where I found the requirement when digging around the repository.
When I saw the requirement in LICENSING.md, I took that at face value. I think that was a fair assumption to make, but I'm still sorry that I got it wrong, especially as this became the top comment.