it takes bad examples of academic English (the writing textbook) and suggests they should be more like bad examples of informal English (the third-grader).

If informal English and academic English are the two extremes on a normal distribution of writing styles, and exhortations to alter your writing style tend to have a smaller effect than their intensity should dictate (because writing styles are well-ingrained), this advice should push academic writers towards the middle of the distribution but not far enough to get into informal areas.