Essential complexity is what program must model based on user needs. Modelling is done for the end user. That's why author mentioned "user says". It's not like "user understands how to describe what they need" rather "we model what user needs". Users will often ask for impossible or contradicting features and can't understand that without our help. I agree that average user will need a lot of help and work to define what they need/want.
Practical example of my abstract description.
User does not want cdn-cached, region-distributed, lambda-backed resizing and validating image services, usage of the latest image compressing formats with fallback for the older browsers with performance dashboats of speed of loading. User wants to see their profile photo on profile page. Showing that photo is an essential complexity as it comes from the end user needs.