That's not what the article is saying, at all. There
ARE general differences in style and word choice between minority groups or women and the average white male in writing. The corpus of training examples used in making this AI grader are at least biased towards the average white male. When the AI grades an essay harshly, it is not saying "this essay by a black woman is bad writing", it is saying "this essay differes from my training set by an aggregate score of ____", then sorts those results and (maybe) applies a curve.
An essay could be different from the reference standard because the standard is an example of good writing and the essay is not. Or it can be different because the author has a cultural, regional, gender, or developed background that imparts a different style than anything in the training corpus. Mistaking the two is very, very bad.