Systemic racism is A problem. You'll never find a word I've written that denies this. But it's not the best lens through which to view every single problem. All this habit does, as I'm seeing plenty of in the comments of this article, is encourage people to stop thinking, because they think saying "white supremacy" answers every single question. Even when a white boy kills three white men who attack him.
Just because I'm saying that white supremacy is not a good way to look at this particular case doesn't mean I'm saying that white supremacy doesn't exist or that it isn't a real problem. When did we lose the ability or will to look at complex situations with nuance?