Those quizzes are completely worthless. There's no way 53% of the population doesn't know that. It was probably worded weirdly or set up in a way that tricked people.
Another one that made the rounds recently was the one where they asked people to estimate what percentage of the population is trans, and the median answer was like 20%. There is no way this captures actual beliefs held by real people, it's a number so unaligned with basic reality that the only thing it points to is a flaw in the test.
My working theory is that a good chunk of the interviewees are uncomfortable being asked nerd shit like this and they just say whatever they think will make the game stop.
That people over-estimate the size of minority groups is a well known research finding [1]
As for the 53% stat, dude just go to Walmart and talk to regular people [2]
[1] https://www.scientificamerican.com/article/people-think-mino...
And people being knowledgeable about what they do day in and day out seems like a good example of people being really good at habituation, but that's not "not stupid." Stupid is when people do the same things day in and day out and then use that as a crutch to say there's no other way to do that thing even when there's evidence in front of them that contradicts what their habituation has led them to believe. I guess you can read the first example as a political statement, but I'm not trying to make one, and the second one applies to everything under the sun, I think.
All that said, I think the author is only saying that when someone uses "most people are stupid" as an excuse but I don't think most folks actually ever say "most" people are stupid. Instead they point out that a decent enough chunk of people are stupid and that's enough to cause some issues.
Disclaimer: I'm not saying I'm not one of the stupid ones. Sometimes I am for sure