College football is filled with subjectivity. The polls themselves are subjective. So yes, any grading is also subjective. I don't think the author would disagree.
I see the grades as generally on-field results relative to talent and expectations.
So a C+ for Kansas suggests slightly above what talent and expectations suggested. Kansas was 0-9 in Les Miles' final season (2020). So while 2-10 in 2021 is still bad, it's an uptick with some moments that provided hope for Leipold and the future of the program.
The D for Nebraska is also reasonable. There's no way 3-9 (and 1-8 in the Big Ten) met anyone's expectations nor was fitting for our talent level. It's still the worst record in Frost's tenure. It would have been an F grade had we not been competitive in our losses.