Just a bunch of random thoughts, after reading the article (I'm neither from Japan nor USA):

This is not to say Japan is immune to industrialized and ultra-processed food trends driving up obesity rates worldwide. Excess weight is a growing concern here, too.

Even homemade treats like karaage can be an issue, too. The underlying problem is the same - it's highly caloric food tweaked to be as tasty as possible, that doesn't offer you a lot of dietary fibre or micronutrients. However at least homemade treats can be tweaked to become slightly healthier, while mass processed food will always contain as much fat and salt as allowed per law.

From what I recall, USA subsidises maize. That would be great if the subsidised maize would go into more "direct" food items (polenta, nixtamal, boiled, etc.), but I bet that most of that becomes corn syrup. Perhaps it should shift the subsidy towards vegetables in general?

I also wonder if the "heavy breakfast, light lunch" culture doesn't play a role. Usually a housemate will have more time to prepare a good lunch than a good breakfast; that usually means just high calories to keep you through the day, but relatively low amounts of micronutrients.