Really? So one MUST refer back to "authoritative" sources? So, if I wanted to write an article on how to repair/modify a telescope mount using processes I implemented/figured I have to dig around and find a source of someone that may have also done such, realizing that I never depended on an outside source for my actions/processes?
If one asks for outside input, then one simply attributes in the article (either in a footnote or when using the actual input itself) the provider of that data in the format of a quote or footnote reference.
Just because YOU may not like the way it's written does not mean that it is written wrong.
Does that user have a normal title? Are they shown as staff? I seriously doubt that a normal user, looking at a post from another normal user would interpret anything as a "forum policy".
In fact, you can EASILY create some prefixes for that node that ONLY the moderators/admins have access to and have one set aside as "Posters Opinion", "Opinion" or similar and then assign that prefix to that post. If said user tries to change it, or does change it several times, then that is a user moderation issue that is addressed in the appropriate manner.