Dude you don't have to white knight for the douchey PhD guy.
not white knighting. it's just personal experience.
Saying shit like that, even if you are really super duper smart, is incredibly arrogant.
you clearly have no idea how draining it can be to deal with idiots. the mental fatigue alone is depressing.
besides they didn't say you were stupid, they said they are free to make you feel stupid. you clearly are dealing with some feelings of inadequacy and felt threatened by that statement.