If you were born in North America, Europe or another developed country, you are privileged. Per the Global Rich List, at an annual income of $13,750, globally you are in the "privileged" 9.9%. That would include most of the US poor referenced in the article.
Maybe the article could have taken a different angle. We are indeed fortunate.
Yep, and that's just those alive today, disregarding the grinding poverty and suffering that mankind has experienced for millennia.
Gratitude, being grateful for what you have and what you have received, is increasingly recognized as an important facet of happiness. People who are grateful and who express this gratitude toward those who have helped them, and who practice doing this consciously, improve their happiness. It is easy to be miserable, regardless of what you have, if you feel that "society" owes you more, more, more.
If the author of this article wanted to help avoid putting people on the defensive, it would have been best to avoid the personal vignettes (which, IMO, helped make the piece much too long for the ideas conveyed. Maybe The Atlantic pays by the word).