My personal take on this: if a person claims to be too smart to be brainwashed themselves, then they are probably not as smart as they think they are.
Kinda reminds me of the people who say that they aren't susceptible to advertisement.
Everyone is susceptible to brainwashing. It's also called "indoctrination". Smarter people (or people with better critical thinking skills) are less susceptible. But no one is immune; if people hear the same thing over and over again, eventually it'll influence them.
Isn't that just another way of saying someone is stupid? I'd say a serious lack of critical thinking skills qualifies as "stupidity".