Nutomic, one of the original developers of Lemmy explains here that overwriting your post and deleting your account will remove the content permanently. Deleting a post individually will however leave its content in the database of the instance, for recovery purpose:

https://github.com/LemmyNet/lemmy/issues/2977

i blocked all news and politics communities so memes and satire is my main source of information, and i have the feeling that i’m still on top of most of what actually matters