I'm an American making half that and saving plenty while being in a mid-sized city (I believe anyone making $120K/year and "isn't able to save anything" is more likely just bad at or unwilling to do money management), but I agree with your point in general.
(The main category where local cost-of-living isn't relevant is travel/expatriation.)
I assume they meant 10k a month household income, and even that is almost twice the median US household income.
Minus taxes, minus loans, minus supporting your dependents (childcare/pre-school being huge), and paying for survival items, doesn’t usually leave you with 40% left over.
My wife and I are closer to twice the median. 3 kids but no loans. We save “plenty” for retirement, but nowhere NEAR 40% (not enough to buy a house in our area in the near future, or to save significantly to help our kids past 18 for example).
And yeah, people are generally bad at managing money, you’re right about that. But it’s also true that most American households have disposable income in the single digit percentages, or no disposable income at all.