Free inmigration is not good. If one person is providing something to the society in one country, that person whould be allowed to remain in that country, whatever the conditions were which caused that person to arrive to the country.
There are two kind of nationalisms: liberal and socialist. It is one condition to allow free movement of people in the country to be called "liberal nationalism". A country which doesn't allow people to freely move in or out must be called a socialist nationalist country. And, yes, it is refering to that kind of national-socialism.
Not talking about what Trump is doing in general, just about what the post is telling.