The article mentions that the paper is under review, but I'm guessing the effect size is small and that individual differences between graders is very substantial. The article states:

> The researchers collected available historical data of all programs, students and assignments on Canvas from the fall 2014 semester to the summer 2022 semester.

Thousands of students X 8 years X lots of assignments per year and you get a sample size so big that it would be hard not to find statistically significant effects.