My suggestion here isn't to have the professors do this: it's for the professors to challenge their students to do this.
> ... they keep some sort of overall concept of learning. This is a pretty god-of-the-gaps-ish hypothesis, and counterbalanced by ...
The author is really missing the obvious here. Learning difficult subjects fundamentally changes how you think about and approach things. People aren't born able to engage in critical thinking or being able to reason algebraically or with an ability to navigate formal logic. That doesn't mean school is the only way to impart such skills, but it is certainly one of the ways.
Of course if the metric you use is "ability to answer trivia" then you are going to fail to capture that aspect.
Citation needed on this. But even if I accept the premise, what percentage of school is leaning difficult subjects? For me it was <5% and the other 95+% was stuff that could be beaten easily with rote memorization. The only classes in that 5% were upper level courses that only students who want to be there take anyway.
Unfortunately any of the obvious metrics you might use to quantify this would seem to be hopelessly biased. The more advanced the degree someone holds the farther above average that individual tends to be in various ways. I doubt it would be possible to control for such large cross correlations.
Perhaps asking people who learn multiple languages could provide subjective evidence since that doesn't have nearly as much of a correlation with other abilities.