Ex: The waiter only hears the first person at the table and then runs off before anyone else can speak. Or, they only bring one thing asked for and forget the rest. Or, they outsource bringing the things to someone else and those people do not bring them, and the waiter never checks again.
From the waiter's perspective, the customer seems unreasonable, but when making that assessment, they are missing information.
Tying this back to the article, people aren't perfect at expressing things. When this design doc situation arises, it really is on the author to figure out why that person objects no matter how it's approached. Fresh ideas are often hard to express and for people to process. Sometimes the objector just has a lingering sense the design isn't right but it's not quite identifiable yet. Obviously, sometimes they are unreasonable, but starting there unless they act like the perfectly logical person in the second example is a bad idea.
TL;DR: in all situations, it's best to try to identify the source of whatever's going on, not immediately assume a person is unreasonable.