Calories in - calories out fails to explain why people consume and expend as many calories as they do. As the series discusses, many calorie sinks such as fidgeting and body temperature are subconscious or autonomic and seem to be used by our bodies to help regulate our weight. And some people clearly have very strong hunger drives.

Clearly something has changed over the past hundred years. Saying that we got richer fails to explain why the rich people of yore, while fat by the standards of their day, are not fat by modern standards. Saying we have gone down hill in terms of sticktoitiveness is not really a verifiable claim and does nothing to offer a solution. So it is absolutely worth further investigating what is going on.

>If she walked an hour a day and ate the same amount

If she does more activity, she gets hungrier. This will pressure more eating. Fighting that is a losing battle for majority of dieters.

Furthermore, consider that the body is a lot more complicated than "calories in, calories out". It can change your hunger level, lower the body temperature, reduce energy spent on fighting disease...