It seems strange that the most humble thing you can do is to assume everyone else is stupid (even if you were to include yourself in to that mix)
If you assume you're smarter than everybody else, you're being arrogant, if you assume everybody else is smarter than you, you're insecure.
Assuming everyone is stupid is the same thing that assuming that everybody is smart.
If everybody is smart, smart must be a common trait, nobody is special.
Someone can still be smarter or less smart than you.
But on average assuming we're all on the same level is not bad.
Except if you assume that everyone is smart, people will think that they are the only one being really smart
The bias works differently in the two cases: of course XKCD got this right (the XKCD guy is way smarter than me)
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.