From Lemmy documentation:
We don’t delete anything from our database, just hide it from users. Deleted or removed Communities/Posts/Comments have a “restore” button.
But don’t take my word for it. Try it out yourself. You can make a comment, delete the comment (your username appears to remain) and then restore its content.
Besides purging your data (we mention how to do this below), we also have an open-source codebase, so you can verify that the actions are actually deleting your data. I could link the source code for that if you like.
Reddit does not have this. When they tell you they deleted your data, its a “just trust us” from an extremely untrustworthy company.