A decentralized system is much more like email than people may be used to. I can ask someone to delete an email and they might even say they did, but it doesn’t mean it’s true.

If I delete content on Reddit it disappears from Reddit, because there’s only one and they control it. That doesn’t necessarily mean they deleted it either, but at least it doesn’t show up on the page anymore. That also ignores that Reddit has been scraped and archived for years and years by third parties, so there is a good possibility it still exists there too. (This scraping could eventually watch lemmy too).

A public forum is a public venue, best to assume the lack of privacy.

Edit: as for the political stuff brought up in the comments, I have said before that I think people need to think about the admins of an instance when they choose to host a community in that instance, that does not mean the project is an issue. The developers have been very supportive of people’s rights to think differently than them.