You're confusing simple with easy. The article is talking about complexity.

From first hand experience I can tell you weight loss is simple. It's just not easy. It's a process in which you learn how you can eat less and/or healthier in a manner that works for you starting from your access to food, habits, schedule, what food makes you feel good or not, etc. It's a lot of work, but the process is simple, you try things and see what works for you.