Closures are not simple. They are a powerful concept, one which introduces a whole new way of thinking about programming compared to what most people start out doing. It's normal for it to take a long time, over many sessions, for the concept to sink in and become a part of your programming vocabulary.

Freshmen year of undergrad, it took me well over a week to wrap my head around this "public" and "private" concept of classes. I had only written C++ that was mostly C with structs and C++ input/output. I had never had to reason in an object-oriented way before, and it was different. It was completely alien to me that I would intentionally "hide" some parts of my code from myself. Now, it's completely natural to me.

What the author did not see was those same developers learn about closures for the first time. When she sat down with them, they had already reasoned with the concept many times. Don't confuse initial difficulty with some inherent deficiency in your mental ability.