That's not how it works in real life from my experience.
I usually assume everyone is stupid, including myself, that's the most humble thing a person can do IMO.
But what actually happens is that other people assume that I am the only one who's stupid, because I'm open about the possibility, while they don't even consider it a thing.
It's a well known human bias, there's almost nothing we can do about it, except train or minds to see others and ourselves as fallible, instead of victims of the system.