Claiming that "leftists" of any stripe are causing an increase in white nationalism is like blaming oxygen for an increase in forest fires.
No, that is a completely invalid analogy. Oxygen is an non-sentient object that doesn't not make choices to act in ways that fuel the fire. An apt analogy would be a person who comes across a forest fire and then dumps a truckload of gasoline and pumps artificial sources of oxygen into the fire, increasing its size and damage, and reducing our ability to control it. No, they didn't start the fire (Damn Billy Joel), but they are making it worse by adding fuel to it.
White nationalists are whiny bullies. Why would any rational person take any stock in the claims of victimhood?
No one is denying that they are bullies and assholes or claiming they are innocent victims. They are people who, like all people, are impacted by and react to their social environment and the ideas around them, including efforts to force them to be hyper aware of what racial category they belong to and efforts to use that category against them. Yes, their own negative tendencies and qualities impact whether they react to racist attacks by being racist themselves. Just like Black Separatists (who are also greatly on the rise BTW, according to the Southern Poverty Law Center), respond to racism with hateful racism of their own. But that doesn't change the fact that racial identity rhetoric and assigning a trait to members of a race due solely to that categorization will be a determining factor for many people in whether they move toward racial ideology and how far they go in that direction.
Also, huge numbers of whites have in fact been victimized, not because they are white but because most victimizations, including those due to our political and economic system, have nothing to do with race, so members of all races get victimized by more powerful people of their same or other races. They are poor, have lousy schools, got screwed by employers, and have no political power. You telling them that their skin color makes them "privileged" and that they actually have tons of power just because the very people that do have power and screwed them over share their skin color is objectively false, racist, and understandably rage inducing. And, that same lack of actual privilege that makes them poor, uneducated, and lacking real political influence is what makes them more likely to react with white nationalism to your racist attacks, condescension, and lack of empathy.
Leftists that engage in "white privilege" rhetoric are also ignorant bullies. Sure, just like many playground bullies, they got bullied first by some asshole, but then they go and take it out on some other kid just because he kind of looks like the original bully, even when that other kid has already been getting his ass kicked by the original bully too. Now that third kid is more likely to become a bully himself and eventually you have a playground filled with nothing but assholes.