Yeah this means that if the post which is reported is six months old, the mods will not check it if it falls under the following reporting criteria.
We generally do not moderate posts this old for on-topicness, substantiveness, multi-posting, and other "cleanliness" issues.
However, it can still be acted upon if it is reported for plagiarism.
It might not seem like a short time but, 6 months is still a long enough time for information on a thread to have attained the much publicity it needs on the forum and should it be something worth reporting, one might have noted and done that. Its half a year of we dont really realise that. Still, I don't think it's something that won't be acted upon if looked up. Its very much up to the moderators on that as we all know.
Still, the rule might not directly ask moderators to act on it but, its not justification that you've escaped some adversities that could come by the message you passed. Rules still remains rules.