If you insist the term "owner" for copyright grants, you have a faulty understanding of copyright. The terms of service, much like software license, only allows for the licensee to do some specific things (in this case, including backups) under certain circumstances agreed upon in advance. Copyright assignment, which is akin to the ownership transfer, is much harder.
> This specifically makes GitHub "the owner" of that data, and not "some third-party" as you originally suggested
This one is my fault though, I've used the "Arctic Vault" as an archival site, but as I later realized it is a Github's archive stored in the Arctic World Archive. So yeah, it's (only) Github that can retrieve the data.
This is a commercial for-profit company, GitHub, taking some code and storing it in cold storage of some other commercial for-profit company, with no one, except these two parties have access to this code. And it doesn't look like GitHub even has the right to do it because it stores it for some purpose other than whatever is stated in their ToS.
I wonder if the whole kerfuffle around Copilot will end up spilling some light on this, too.
> This specifically makes GitHub "the owner" of that data, and not "some third-party" as you originally suggested
This one is my fault though, I've used the "Arctic Vault" as an archival site, but as I later realized it is a Github's archive stored in the Arctic World Archive. So yeah, it's (only) Github that can retrieve the data.