You skipped over the fact that "bless their heart" itself is (or at least used to be, before it became too well-known to really be a “secret” any more) precisel

You skipped over the fact that "bless their heart" itself is (or at least used to be, before it became too well-known to really be a “secret” any more) precisely such a secret code. (Like, probably, most “HR talk”.)