It's bad science. They did not do any work to rule out other factors that are correlated to altitude but that are not the watershed. Off the top of my head: lower oxygen levels leading to greater cardio-vascular fitness; affluence - in the US I would not be surprised if richer people live in the mountains, while poorer people live on the rivers like Mississippi; activity level -- more active people go to the mountains to do their mountain sports and so on.

If you have thing X that you think might be partially caused by A, B, C, D you need to rule out B, C, D, before you say that A causes X. Otherwise we're back to 5G causing covid because if you draw a map of cell towers, it lines up with a map of hard-hit covid areas.

For a concrete example, I'm reading a theoretical neuroscience paper on dimensionality. The authors found an interesting relationship, but they spend a lot of paper on showing that 3 or 4 different known things are not the cause of this interesting effect. They do this by measuring the extent to which the other known effects explain away the effect. When all that is done and a significant effect is still left, then they conclude that they've found something new and real.

Humans are wired/encultured to find narratives more interesting than facts. So the watershed thing is a well-written narrative that makes it convincing but does nothing to establish the truth of the matter.

While there are many many factors affecting obesity, I am still convinced that the most significant factor is that people are basically delusional about how much they actually need to eat and how much energy is actually in various foods. I could be wrong, but I've seen no rigorously done science that convinces me otherwise yet.

I remember what it was like in the '80s. People just ate a lot less at every meal -- our portions have become ridiculous. If you work with immigrants (e.g. Russians). Look at some trim people and see how much they bring for lunch. One small sandwich, an apple, maybe a small salad. That's it. Pretty much any store or restaurant-bought meal here is going to be much more calorific than that.

The whole "low-quality" foods thing strikes me as new-age marketing. There may be something there, but again I believe the overriding factor is that most people don't understand how little they need to eat when they're driving and sitting all day.

EDIT> Also, in the '80s it was basically a luxury to eat out. You weren't going to go to Starbucks and have a cake in a cup for $5

EDIT2> I've watched smart people struggle with weight-loss and overeating my whole life. The excuses and delusions would not be surprising coming from a heroin addict. When I was 210 pounds after being 190 in university, I looked fat to myself. When I was 230 pounds after being 245, I looked slim to myself. This tells me that I, and the other people I have observed cannot be objective about food and weight. So I am very skeptical of non-rigorous opinions or popular-press opinions on the matter. I over-eat when I feel sad about having eaten too much -- that's literally crazy, and I've observed this craziness in everyone else I know who is fat! Food is such a social thing that we have person after person enabling and affirming our own bad habits. Grandma hopefully would never offer you a hit of heroin if you told her you were trying to get sober, but she'll totally offer you a big piece of pie if you tell her you want to lose some weight. Other people around the table will tell you "It's okay -- just this one time," and if you strenuously object you're being rude. We are delusional about food.

EDIT3> In summary, we are extremely biased to find any reason other than we eat too much, and the quality of a lot of the writing and even science in this area is pretty low and compromised by financial motives.