For a long time, before the current snafu, editing your comment before deleting it was the only way to expunge it. Probably no real guarantee, because archiving the database was already a thing, but applying the edit used to also update the archive, supposedly. If it was just delete, then it would linger after you deleted your account. Modifying your comment or post, then deleting your account, that leaves the modified comment even though you now can’t go back and make changes or delete it. The suggestion I believe is to modify the content, then just delete the account. Deleting the comment isn’t enough supposedly, because some people have said that their deleted comments have been restored.