I can understand that some racists would believe that. It’s difficult to believe that they would articulate it outside a trusted group of fellow racists. It’s totally incredible that a U.S. congressman would be the one doing the public articulation. I just can’t imagine anyone that clueless.
For the record:
A supremacist is one who thinks one group is superior to another. Therefore a white supremacist is one who thinks the white race is superior to others.
A racist is one who believes that one race is superior to another.
In other words a white person who is a “racist” is precisely the same as a “white supremacist,” by definition.
I’m going to go out on a limb here and say that might be why white supremacism is recognized as offensive.