OP said it was unhelpful to consider it one country, not that it was untrue to consider it one country. I think this is true in the same way that it is unhelpf

OP said it was unhelpful to consider it one country, not that it was untrue to consider it one country. I think this is true in the same way that it is unhelpful to talk about the mean net worth of 10 individuals in a room if one of them is Elon Musk. And, in fact, treating the US as "different countries" is exactly what the author does - by pointing out that it is low-income Americans who actually suffer the burden of lower life expectancy rather than an across the board radical lowering of life expectancy for all Americans compared to other rich nations.