Absolutely not applicable when brought down to the census tract level in large cities. Compare, for example, the percentage living below poverty line in Manhattan (density of 27,227.1/km2) with the percentage living below poverty line in Brooklyn (density of 14,037/km2).

> when brought down to the census tract level in large cities

Which is exactly the same with any population heat map when you go down to the census tract level in large cities.

In other words, it is completely applicable.