Stupidity is being presented with a problem and an associated set of information and being unable or less able than others are to find the solution. That's literally it.
But it's an incomplete definition - Cipolla's definition is "someone who causes net harm to themselves and others" and is unrelated to IQ.
It's a very influential essay.
So they just got Cipolla's definition wrong, then. It looks like the third fundamental law is closer to "a person who causes harm to another person or group of people without realizing advantage for themselves and instead possibly realizing a loss."