[googlryas]

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.