So while that
may be correct, it's absolutely not what this bug proves.
This bug shows that when you delete a file on iOS, it does not immediately get removed from YOUR filesystem. The 'important questions' you raise are predicated on a misunderstanding.
In particular: 'why is Apple retaining these photos?' - Apple isn't (or at least, this bug does not show that they are). Apple reimported photos that were still present on your filesystem after you had deleted them.
The question of why they're still present on the filesystem is a good and fair question. Some folks are suggesting it could be to do with these actually being copies of the originals (which is perfectly possible—as far as I can tell, copies are created when you share and when you edit) which Photos lost track of, or some engineer forgot to consider when deleting. Or it may be a performance optimisation to simply dereference the file so that the user has a silky smooth experience, and allow a background job to eventually clear up the file system, and potentially that background job crashes from time to time, whatever.
Your questions on data privacy are not founded in fact here. There are some legitimate questions that users should be asking Apple here (why does Photos tell me it's deleted a file when it's still on my device is a pretty big one).