The article actually starts by acknowledging that there used to be a feature called "Unsend", and that the issue being reported here is NOT about that. From what I understand, Twitter keeps storing messages that the _recipient has deleted_ even after the _sender account was closed_. I don't think it would be an unreasonable expectation to see these messages deleted from Twitter storage.

The question IMO is, at what point should you stop storing a message that no user can actually access anymore? That data is not good to keep for anyone, since both sender and recipient expressed a desire to delete it.