I'm still new to how lemmy works exactly but why does trying to delete a post not actually remove it? It's still fully viewable so are you not able to delete posts on here at all? I'm a bit confused.
Just FYI, coming from a Thunder dev, the full contents of deleted and removed contents are available via the API, and it's up to us to hide the contents. It's easy to miss use cases (like another user mentioned you can see deleted comments by replying in Sync; that's just an (easy) oversight by the dev). I'm glad to hear that they won't even be available in the API starting with 0.19.4!!
This is addressed in the upcoming Lemmy release 0.19.4 where contents will no longer be included in API responses. Until then it's up to clients to actually hide it. Content is kept for a few days to allow you to undo deletion, but you can also edit your content before deleting it to remove that. There is also a scheduled task running once a week I believe that will replace contents of deleted comments with something like PERMANENTLY DELETED.
Regardless, as Lemmy is a public platform, you should be aware that people may be storing this information on linked platforms regardless and may not respect the edits/deletions at all.
Others don't see the text of the comment though, it just says comment is deleted.
To answer your question, basically for Federation reasons (if I had to guess).
Its the same way with when you block someone, you're not blocking them from the seeing you, you're blocking yourself from seeing them. They can still respond to your comments and you wouldn't know they had.
With the Sync app you can still see the comment if you hit "reply", as the reply view has a top box for reading the parent comment and making text selection and for some reason it retains the comment contents. Just an FYI