A more verbose quote from that paper: "What makes nations unique is that they are collectives united by shared cultural features (myths, values, etc.) and the belief in the right to territorial self-determination. Thus, nations are groups of people linked by unifying traits and the desire to control a territory that is thought of as the group's national homeland. The belief in the right to territorial control is central to distinguishing nations from other collectives. Many groups hold common myths, values, and symbols (e.g., religious groups, ethnic groups, or even professional associations). But nations are not just unified by culture; they are unified by a sense of purpose: controlling the territory that the members of the group believe to be theirs."
This is almost identical to the use Bret Devereaux uses in the artice.
It is not any shared cultural feature (but e.g. a common origin myth would fit), and the people have to feel a sense of purpose, of common past and future relative to their homeland. As stated in the article, that certainly fits Native American Nations, but not the US as a whole, as Barrington discusses in the next paragraph, "Nation: Misuses".