I also believe anyone can learn anything, exactly as you said!

But I don't find the study about grade distributions a convincing argument for that, unfortunately. Grades are a measure of student's ability to apply consistent effort on assignments and effectively study for an exam, often collaborating with a network of other students to find the correct solutions.

A determined student can earn a good grade. A determined student can master the material. There is overlap, but not enough to use a premise that Grades<=>Ability

A more convincing study would find a way of evaluating their individual ability to solve problems related to what they are supposed to have learned, not their grade in the course.