See, here is where I personally disagree. I am of Polish heritage, and I was born in Poland. Half of my family is of Jewish heritage and from Warsaw, so most of my relatives fought in the Warsaw Uprising and/or died in labor and concentration camps during World War II. The rest of my family, which was from villages in western Poland, were conscripted by the Soviets and pushed into labor camps, or fought the Germans in woodland rebel groups. I am proud to be Polish and I consider my Polish heritage important to me for a few reasons.

Poles traditionally have a strong sense of nationalism, due to the unique history of Poland, particularly the Partitions of Poland, which effectively wiped it off the map for over 200 years. During that time when Poles had no country, they developed a powerful nationalism, based on common cultural and linguistic heritage, which was reflected in the arts and literature of the time (much of which came from Poles who emigrated to France). Chopin, for example, who was one of these Polish nationalists in France, composed extensively about Poland.

Look at the first lines of Pan Tadeusz, another product of the Polish emigrates in Paris, published in 1834 and set in the partitioned Polish-Lithuanian Commonwealth:

Litwo! Ojczyzno moja! ty jesteś jak zdrowie. Ile cię trzeba cenić, ten tylko się dowie, Kto cię stracił. Dziś piękność twą w całej ozdobie Widzę i opisuję, bo tęsknię po tobie.

Roughly translated as:

Oh Lithuania! My fatherland! You are like good health. How much you are to be valued, only he will discover, Who has lost you. Today your beauty in all its splendor I see and describe, because I yearn for you.

Consider also the Polish National Anthem itself, written two years after Poland was partitioned:

Poland has not yet perished, So long as we still live. What the alien force has taken from us, We shall retrieve with a sabre.

March, march, Dąbrowski, From the Italian land to Poland. Under your command We shall rejoin the nation.

Nationalism kept Poland together as a nation, even when it was without a country. It was so strong that even when Poland was finally given country status in 1918, and plunged into war just 30 years later, it would not let itself be taken under foreign forces (German or Soviet) willingly. Hundreds of thousands of civilians died in the Warsaw Uprising; pre-pubescent boys fought and died not for the 30-year-old country, but for the nation. Poland was one of the bastions of resistance in the USSR and led, in a large part, to its collapse.

When I consider my heritage, I think not just of the fact that my relatives fought and died in defense of it. I think of the hundreds of years of history where people felt that Polishness - Polish culture, language, and nationalism - was worth preserving against difficult (and often deadly) odds. I feel a link to every other Polish person because we share these common bonds: a country that was a powerful force in history since the 10th century; that was more than once threatened by outside forces, which cemented a national identity due to risk of losing it; and which, against all odds, spared my grandparents so that I would be born.

I posted the article in the OP because I do agree that racial categorization is imperfect and I am interested in discussions about whether classification is even useful and if so, how we should do it. However, I understand pride based on one's heritage and will continue to identify strongly with my Polishness and other Poles.