that racism means “the idea that there are races”.
I think you would need to specify "the idea that there are biological races" for that to even remotely work and calling yourself white doesn't imply you believe in biological races. Like someone saying they don't worry as much about cops pulling them over because they're a white (male driving a truck through rural Texas), for example, is talking about the social construct of race and how that influences their normal interactions compared to their darker skinned pears in a way that criticizes racism rather than reinforces it.