The study he's referencing took into account exercise as well: http://www.sciencedirect.com/science/article/pii/S1871403X15...
That study is indeed interesting. I just skimmed through it and read the abstract, definitely interesting.
The question now is, what is it? Could it be that the difference in food – not just caloric value of intake – has an influence? Did people start eating more prepackaged foods? If yes, what specifically influenced it? Why did it not influence Europe nearly as much?
Compared to driving everywhere, sitting at work, etc...