Once you start to think of mistakes as deterministic rather than random, as caused by "bugs" (incorrect understanding or incorrect procedures) rather than random inaccuracy, a curious thing happens.

You stop thinking of people as "stupid."

I agree with the general thrust of the essay (that broad, fuzzy labels like "bad at" are more useful if reduced to specific bug descriptions,) but I'll note that being aware of the specific bugs that cause people to make the mistakes they're making does not stop me from thinking of people as stupid. If a person's bugs are numerous, obtrusive, and difficult to correct, I'm going to end up thinking of them as stupid even if I can describe every bug.