The line of thinking progresses like this - it starts at ignorance, then it comes to the showcased "there are no stupid people" partially because of the still remaining ignorance in the "unimportant" humanities areas usually "STEM and economics is all that's needed to be smart!". And partially because of the combination of self-censoring and social virtue signalling "Look, I'm super duper extremely tolerant and avoid even a slight mention of the "bad words", please praise me!". And then, rarely, a person understands that a) there are no "bad words" and opinion of others is more or less worthless with the exception of friends and family, and b) a lot of people are actually stupid in a clear biological or medical sense. Like for example, the oft incorrectly remembered IQ score. It is quite openly in the definition of that score, that score 100 is around the median. And word median implies that there are not only higher scores than 100. but also lower too. It's really obvious, and after virtue signalling is no longer in the picture, it is very easy to explain.