It is an opinion piece. It is the opinion of an author who looks at something, has an ideology etc and creates a thought of it and decides to write an article about it. DM is providing a platform for that person to air their view, as long as it does not infringe grossly on another persons rights. Whether it is right or wrong depends on the reader. If the reader chooses to believe it, it is up to them.
What a person sees as wrong, may seem right to you. What is a lie to you, another sees it as truth. 
So, if DM were to place a disclaimer, what would or should its wording be so as not to make the reader of the piece be made biased for or against it before they have read it, that it does not appear that DM is interfering in the authors ability to express freedom of speech, or is not appearing to censor by putting off potential readers?