As a counter argument, what would persuade me to set aside my differences and accept your view if you don't first make an effort to show me we agree on the 90%? Why would I consider your 10% if you don't have the time to consider my own? How can we each understand the 10% where we're different, and feel like there's value in discussing the compromise of that 10%, when we don't start first with understanding the 90%?
In a nutshell: why bother putting the cart before the horse when we have no idea what the 90 and 10 are and just start talking about the 10 as if it were 100%?
I think you are assuming you are sitting down with someone you already both know each other very well (you've already long ago established that 90%), but the author is essentially talking about NEW discussions with NEW people.