There is more to most discussions then simple facts. There is the assumption in the text that if we only understood the material world perfectly, we could all agree in the proven objective truth about anything. This simply ignores all of human emotion and the way that we live together in society. If we all could just be perfectly rational the world would be perfect without any conflict. But we are not. You simply ignore that humans have emotions and live in societies which live by certain morals and ethics. Those are not simply utilitarian. Even if a complex subject would become graspable in every little tiny facet and only facts remain, those facts have to be evaluated. The evaluations about the relevance of facts are subjective by definition.
Good point. Even in a universe in which everyone's values, emotions, motives, etc. were completely transparent there would still be conflict because what is in one person's interest isn't necessarily in another's. I have failed, at best, to define "disagreement."