It literally does not, except in the sense that it ran the method on a list of countries. From section 2.3:

> Overall, 8 out of 10 countries with the highest undercount ratios in the World Mortality Dataset at the moment of writing demonstrated statistically significant underdispersion in our analysis.

Translation: they ran it against a list of countries with known "undercount ratios", and found that 8/10 had high scores. Also:

> The correlation of underdispersion index to undercount ratio was 0.40 (Figure 3).

This is a poor overall correlation, and they do no sensitivity / specificity analysis so you can't jump to the conclusion that a score is meaningful. They make the argument that it works better for the countries with the highest undercounts...but if you know the countries have a high undercount you already know the answer by other methods. And even there, 80% sensitivity isn't that great.