We weren't talking about ownership, though. We were talking about reliable deletion of data. Ownership has little to do with whether you can reliably expect that your data has been deleted, access does. If anyone can access the data, they can copy it.
Of course, having only one server that you are posting ypur data to makes it less likely that it will still be living anywhere else when you want it deleted, and that is an advantage of Pubky, but it comes with the disadvantage of not having redundancy: less censorship resistance.