If you're asking how people can hold contradictory ideas... then I'd invite you do some self reflection. You probably hold some. We all do. Striving towards clarity of thought takes a while.

As for white supremacists in specific, let's take David Duke and Richard Spencer for example. (As you agreed above they were white supremacists) Both of these people claim to believe in science and reason, yet also believe in the superiority of the white race.