That is not defence, that is actually collectivism and authoritarianism without respecting human rights, equal treatment and freedom. What nationalist do is to use myths and identity politics around those issues in order to promote fear and collectivism, and corruption as well. Even on basis of racism and culturism. So they are not defending anything expect of their own strange fictions. Because things as culture and identity are based on social constructions and affections, which are changing all the time. The way you are presenting culture here is something else than what science and research has concluded many years ago.
Iceland is one of the most globalized countries and has quite open borders and immigration, even if the immigration levels are small because the country itself i small. This is why for example 3 million people from Africa cannot come to Japan because there will simply not be enough place.
Hungary and Poland have regimes with nationalist sentiments which are conducting corruption and statism. This is the especially in Hungary where the government is quite corrupted. government's policies there that are keeping many wages low and workers have problems to negotiate wages with their employers because the Hungarian government is a state-capitalist one. So the Hungarian government is not "defending" their country, they are defending their own positions. If you like corruption and cronyism, be my guest. Also, nationalism in Europe is historically and politically dangerous because it goes against ideas of European constitutionalism, democracy and citizenship.
Yes, I favor open borders and global free movement on basis of liberal/libertarian ideas. As with the case of Bryan Caplan, who for example shows that immigration from the developing countries does not lead to developed countries becoming worse. That is the point of libertarianism, that one does not care about culture, race or gender. One does not see people as being "imported" or "exported". One supports freedom for all humans simply because we are human beings with our freedoms, rights and sovereignty.
Also, those who are or who are seen as globalists also have their identification and affiliations with cities, states, regions etc.
https://conatusnews.com/a-borderless-world/