They are of the believe that there is one correct viewpoint and it is their own. So your observation is correct. Their worldview is absolutely not compatible with moral relativism.
This is an unfair generalisation of libertarians. Some libertarians think that their view is the only correct one but not all of us do. I'm sure the same could be said for some authoritarians and some people in between.
I am a libertarian and I wouldn't go as far as to say I am a moral relativist. I believe I am right on the whole about moral issues but I accept that I may be wrong and keep an open mind to other viewpoints. I also acknowledge that there are many difficult grey areas in morality.