Earlier quoted context omitted.
Well the Axis powers from World War II are the most obvious demonstrations of nationalism begetting authoritarianism. Germany, Italy, and Japan were nationalist in the extreme. And Italy from that time is such a clear example that it's basically the canonical example used to teach how fascism emerges. Contemporary examples include the Philippines, Hungary, Poland's Law and Justice Party, and arguably Russia, Turkey a…
But that would only be something nationalism signaled if the converse weren’t also true — eg, totalitarian states like the USSR, CCP, etc. Those also had: - grievance narratives; - a tendency to divide the world into loyal citizens and enemies; and, - use the above to justify authoritarian powers. You haven’t shown that nationalism played a particular part in that cycle; just that it also happened in nationalist stat…
I think the major difference in their respective cases pertain to the ideological dynamics of the particular strains of communism that manifested in those countries. What they lack is a fixation on the purity of national heritage as a primary source of moral truth and a foundation for a self conception. Instead they tended to regard themselves as part of universal, international struggle and understood conflict in economic and ideological terms. What they had in common was the sense that conflict with this chosen enemy necessitated authoritarianism.
There's more than one path to authoritarianism, and they overlap. Different mechanisms don't disprove one another, they exist side by side.