Overwriting or editing a post will almost certainly just create a new revision of it in their database. All these tools work on the basis that doesn't happen, but that deletion is a flag rather than a drop, which is pretty inconsistent. The reality in every large cms/forum software I've used is that revisions are the norm.

Reddit have the ability to keep all revisions of all posts made on their servers - they may even be legally required to do so. If a police agency requires evidence relation to a post on reddit linked to terrorism, they're not going to be pleased if it is so easily eradicated. Some people think that GDPR gives them the absolute right for their data to be removed - but not if it conflicts with other laws and legal requirements, and even sometimes commercial interests (See "legitimate interest")

Bottom line is - if you don't want something to potentially exist forever, don't post it on the internet and pass control of it to others.