Well, the main point of that article seems to be that once someone has been overweight for an extended period of time, it becomes really hard to get to and stay

Well, the main point of that article seems to be that once someone has been overweight for an extended period of time, it becomes really hard to get to and stay at a lower weight. So, yes, it suggests that an increase in physical activity isn't an "easy" solution. However, I don't think it affects the point the article is making. People don't wake up to find themselves obese one day. It takes some time and effort to get there. And I would argue one of the biggest impacts on gaining weight is a lack of physical activity. Not to mention that from my experience, avoiding physical activity has a self-enforcing effect. The more you avoid moving around, the more you want to avoid moving around.