They probably do have copies of deleted posts/comments, also copies of the original for edits. I don’t know of a single company that easily allows users to “really” delete something.

Database backups cannot easily be edited, for example.

I was able to delete all my posts/comments without issue (My reddit username is different from this one fyi).

As you stated, the subreddit could gave been private. He also may have been viewing a stale cache.