What is meant is that he is about to say something obvious, but which he feels should nevertheless be pointed out.
One does not express the obvious in polite company as this suggests the audience is unable to see the obvious. The author acknowledges the rudeness of his statement, and asserts that it had to be said nevertheless.
In other words: "I apologize for pointing out something this obvious, but..."
"At the cost of" or "at the risk of" means that the author realizes his statement comes with liability of some kind and is willing to make it nevertheless.
"At the cost of being rude, I would like to point out that...
Which is why the word "truism" used:
Truism: "A truism is a claim that is so obvious or self-evident as to be hardly worth mentioning, except as a reminder or as a rhetorical or literary device."
That said, adding "or truism" to the sentence means that the author expected his readers not to understand what he said, which is fairly bad form in my opinion.