There was sensitive information in the document that happened to be 100% of the contents, this was redacted. They now cannot be sued for not responding.
Is it a defensible position to assume that 100% of a document was excised as sensitive data? Since the subject of the memo itself is announced clearly, are we to believe that the presentation of the term "message" which almost certainly occurred one or more times in the body text was sensitive? What about harmless pronouns, conjunctions, articles, etc?
If a citizen were to reply to a government request of information from the government in the same manner, few would be surprised at legal consequences (penalties, incarceration) as a result. Actions such as this, in response to a legitimate request from the public allowed under law, can only serve the purpose of undermining confidence in the rule of law.