The paper does not say that professors are only moderately correct about what mistakes their students typically make, just that their personal experience does n

The paper does not say that professors are only moderately correct about what mistakes their students typically make, just that their personal experience does not align with the results of the cohort used in the study. A professor may be perfectly correct at a local level but globally only moderately correct. The contextual knowledge of the professor is not acknowledged in this study (curriculum, course level, teaching style, problem set, student background, lecture notes, textbook choice, compiler, IDE) nor are the professors own students who may only come to the professor or TA with a certain subset of problems etc.