I think you're missing the point of the article. It's not claiming that the mere existance of unhealthy food is what's causing obesity; it's the societal norms and behaviors developed around food (and exercise).
Of course ultra-processed food also exists in Japan. Same for sugary drinks, sugars and sweets. But it's undeniable that there's a culture of acceptance of junk food in the US, and a confusion that some of it "must be good for you" -- e.g., fruit juices, cereals.
There's so many historical differences between Japanese and American societies that could be part of the cause -- many of them listed in this thread.
To give you one example: breakfast in the US is typically full of carbs, sugars and fructose: cereals, fruit juices, bread, honey, maple syrup. There's a common belief that they're healthy, and you need that "energy punch early in the day to keep you going".
That couldn't be further from the truth. All this carb and sugar-heavy meal does to your body is to give your metabolism a glucose spike, followed by a rush of insulin, and eventually a crash [1]. This roller coaster leaves you hungry and craving soon after. Rinse and repeat for 30 years (together with other bad habits), and you're one stop closer to obesity and diabetes.
The American culture of "snacks" is another example: we're inundated with granola bars, dried fruit, veggie chips -- all "organic", "natural", "full of vitamin". Portion size is another one; go compare to portion sizes in the US vs. Japan. Or the common habit of eating fast and until full, while in Japan is 'eat slow, until 80%'. The list goes on.
Now, you need a social scientist to understand WHY it happened this way over the past 70 years. It may have been the food scarcity during WWII, or how baby boomers grew up, the rapid industrialization of American society, or a myriad of other confounding factors.
Whatever the case, it's clear that this is wrecking the metabolism of millions of Americans, and it'll get worse before it gets better.
ps: I'm using Americans instead of Western society, because 1) that's the reality I'm familiar with, and 2) I think this is more prominent in the US than in the Western societies at large.
Source: born and raised abroad, but living in the US for the past 20 years, with a Japanese-descent spouse. This gave me some perspective on how different cultures develop a relationship with food.
[1] Glucose Revolution, https://www.amazon.com/Glucose-Revolution-Life-Changing-Powe...
Let people buy CGMs without a prescription, increase education about healthy habits from the point of view of minimizing glucose spikes [1][2], and the importance of metabolism for everything, including physical and mental health [3].
Yes, diet and exercise helps, but what you eat is also important, as well as when, and in what order. Once you learn about it (and have a CGM showing the effects in your body in realtime) you can't unsee it.
[1] Glucose Revolution, by Jessie Inchauspe https://www.amazon.com/Glucose-Revolution-Life-Changing-Powe...
[2] Brain Energy, by Dr. Christopher M. Palmer MD - https://brainenergy.com/
https://www.psychologytoday.com/us/blog/advancing-psychiatry...