You're of course right that race is a construct. The reason that people were outraged about Whoopi's comments is that the Nazis very much believed Jews were a race, an inferior one at that, and then set out to destroy Jews so that the "master race" (Aryan) could dominate. Obviously, the Nazis were wrong about race, but their actions were based on their misunderstanding of race, so it was "racist" if that term is defined by seeing one race as lesser than another. (Even though yes, race is a construct. The dumb Nazis didn't get it).