Consider the phrase "We live in a democracy culture": You can practically assume that everyone the conversation believes democracy is the correct way to govern, and NOT believing that would be considered very far outside of the overton window. Even as Democrats and Republicans fight over voter ID laws, both sides claim their view is more true to democracy.
So similarly, while racism exists, we do not live in a culture where it is the norm to openly state that "White people are superior." Yes, white nationalism exist and it is a nasty idealogy, but the fact that they have to airbrush their ideology in "ethnostatism" is a sign that openly expressing white supremacy is an unmarketable idea.
This is what I take the phrase "We don't live in a white supremacist culture" to mean. It is a very different claim from "racism doesn't exist". However, I think a more empathetic thing to say would be something along the lines of "White supremacy doesn't define our culture. We live in a nation where this ideology is openly condemned by the vast majority people on the left, center, and right. However, that doesn't mean it is completely absent, especially in our past, whose effects still linger on today."