By this metric some three-quarters of Americans have poor impulse control (cf. https://en.wikipedia.org/wiki/Obesity_in_the_United_States#/...), so I think it is actually the other group that is exceptional.
There are many other nations that reached comparable prosperity and food abundance at similar times, yet don't have even comparable obesity rates. An extreme case is Japan, at 3.5% obesity rate maintained largely though willpower and cultural norms, by literally firing those that are too fat, or in any case applying strong social pressure and mandatory counseling.
1. There are other differences in Japan -- most obviously a very different diet
2. Obesity rates in other countries are catching up to the United States.
3. The first two points call into question your claim that Americans are uniquely lacking in willpower.
4. I don't want to live in a society where people are fired or ostracized for being fat because it is cruel and inhumane.
5. The US has a $66 billion weight loss market, suggesting many Americans do want to lose weight (cf., https://www.prnewswire.com/news-releases/us-weight-loss-mark...), but I'll leave it to the oracles to decipher what you mean by "really" want.
Still bad but not as bad.
It's also important to point out a strong self selection bias in such studies: we mostly deal with those who were overweight and needed to start a diet, so they probably had low impulse control to begin with. An important part of population with better control and similar cravings might have started their willpower exercise after gaining the first few pounds, it's disingenuous to discourage everybody using statistics applied to those who have a trackrecord of failure. If you gained weight by simply not caring about your weight (depression, cultural norms etc.) then you might stand a much better chance of success when you start to care.
The testimonial sounds like a clasic case of artificially lowered basal metabolic rate by crash dieting and associated loss of lean body mass. Such people need high resistance, mass building exercises, not catabolism-inducing aerobic. She might live a very long life tho.