"White Nationalism" is a term chosen for its psyop / propaganda value. It's effect is to imply that every white who is a nationalist is a white supremacist.
The best way to counter this is to first ask them "what does 'white nationalist' mean?" Then, when they say "believing that whites are inherently superior", ask them "then why not just say 'white supremacist'"?
Just like "Black Lives Matter." The term itself, when used as a label, disarms the opponent. Because, to oppose something with a name like that suggests that I oppose the sentiment expressed by the name - i.e., that black lives don't matter. It's absurd.
It's as if I were to start a nonprofit called "Joe Is A Really Good Guy". Then, I go around to ask for donations, and when someone turns me down, I get in his face as an aggrieved victim and shout "what, don't you think Joe's a good guy?" Classic example of bullying.
Identify the propaganda. Call the propaganda out. Destroy the propaganda.