I think it means this. Sometimes when people disagree about something, they either hide their true goal/objective or they are so close-minded that they can’t even begin to understand the other person’s viewpoint. But other times, people are honest about what they want and why they want it. In that circumstance, two people can still disagree, but at least it is “honest disagreement.” Because the people are being honest about what they want and why they want it, there is a better chance for persuasion or compromise than there otherwise would be. So, “it’s a sign of progress” — meaning that the chances for persuasion or compromise are better.