The author of that is wrong to conflate nationality and ethnicity. A nationality is a much more general concept. Because most people in the US would list "American" as their primary group identity, and because the people who don't aren't off in their own geographical areas, the US is more of a nation state than it is anything else. Now, you shouldn't throw around the term nation state unless you deliberately mean to exclude, say, Kenya or the old USSR - but that's a different matter.

EDIT: Or compare the US and the EU. The typical New Yorker regards people in Florida as the same sort of person as them, so the permanent fiscal transfers from New York to Florida don't spark the sort of outrage in the US that permanent transfers from Germany to Greece do.

Uncited articles on "wisegeek.com" -- a good place to learn the definitions of words and terms.