I may be mistaken, but I think it means that one can delete structured data only, because it is owned data. You can only delete what you clearly own, and immutable, because of deduplication, could belong to others as well as oneself. The only way to for the network to tell is if it also tracked everyone who uploaded that chunk and track with permissions to delete. Not a good thing.
Edit to add: one can always effectively delete structured data that one solely controls simply by updating to contain nothing but its shell. I'm pretty sure the point in question refers to supporting the ability to simply do away with it altogether, all traces.