What they really mean, part 2. New definitions.

na·tion·al·ism
noun: nationalism
1. patriotic feeling, principles, or efforts.
synonyms: patriotism, patriotic sentiment, flag-waving, xenophobia, chauvinism, jingoism
"their extreme nationalism was frightening"
2. an extreme form of this, especially marked by a feeling of superiority over other countries.
plural noun: nationalisms
3. advocacy of political independence for a particular country.

na·tion·al·ist
noun
1.a person who advocates political independence for a country.
"a Scottish nationalist"
adjective
1.relating to nationalists or nationalism.
"a nationalist movement"

I did an exact word-for-word search on your comment, that doesn't come from any reputable source. Mater-of-fact, it came up with a suggested search result of (extreme/extremist) nationalism. Which, quite frankly, makes you dishonest.

I checked. My results come from Google (Google dictionary I suppose.) I used Google to search for "nationalism" and "nationalist" separately.

Dictionary.com
nationalist :
noun
1. a person devoted to nationalism.
2. ( initial capital letter ) a member of a political group advocating or fighting for national independence, a strong national government, etc.

nationalism :
noun
1. spirit or aspirations common to the whole of a nation.
2. devotion and loyalty to one's own country; patriotism.
3. excessive patriotism; chauvinism.
4. the desire for national advancement or political independence.
5. the policy or doctrine of asserting the interests of one's own nation viewed as separate from the interests of other nations or the common interests of all nations.
6. an idiom or trait peculiar to a nation.
7. a movement, as in the arts, based upon the folk idioms, history, aspirations, etc., of a nation.

That's quite a bit different.

This probably is a lot closer to where the meme is getting the context from.

Merriam-Webster : (dictionary online)
In U.S. usage nationalism is now perhaps most frequently associated with white nationalism, and has considerably negative connotations.

Some of us imagined that we dented the nationalism, hatred and racism that roiled the world in the first half of the 20th century.
—Jeanette Friedman-Sieradski (letter to editor), The Times-Tribune (Scranton, PA), 12 Mar. 2017

And while coded appeals to racism or nationalism aren’t new—two words: Southern strategy—overt calls to temporarily bar Muslims from entry to the United States or questioning a federal judge’s impartiality based on his Mexican heritage are new.
—Jim Rutenberg, The New York Times, 8 Aug. 2016