What is significant about Stradlater telling Holden "don't stick all the commas and stuff in the right place"?

In Chapter 4, Stradlater asks Holden Caulfield to write a descriptive composition for him. When Holden halfheartedly agrees, Stradlater says: "Just don't do it too good, is all. That sonuvabitch Hartzell thinks you're a hot-shot in English, and he knows you're my roommate. So I mean don't stick all the commas and stuff in the right place." This gives Holden "a royal pain." He comments that "He wanted you to think that the only reason he was lousy at writing compositions was because he stuck all the commas in the wrong place."
A sharp English teacher could tell at a glance, of course, that if somebody like Stradlater turned in a really interesting and original composition with all the "commas and stuff" correct, it would mean he had plagiarized it or had it written by somebody else, most likely his talented roommate. This little incident is an interesting insight into human nature, but its main purpose is to add further evidence that Holden, though a dropout and a misfit, is capable of writing a novel like The Catcher in the Rye. Stradlater acknowledges, in a left-handed way, that Holden is a very good writer and also tells us that the teacher named Hartzell (who must be an English teacher, judging from the assignment) "thinks you're a hot-shot in English." We also learn that two other teachers, Mr. Spencer and Mr. Antolini, express high opinions of Holden's talent as a writer. In addition, Stradlater's concern about the commas being in the right places indicates that Holden is not only good at self-expression but that he is fully competent in such "stuff" as punctuation, spelling, and capitalization. So the reader can feel assured that this sixteen-year-old boy (and not the invisible author J. D. Salinger) is solely responsible for every word in the story, as well as for the quality of the grammar, spelling, and punctuation. We have the illusion that we are actually reading the work of a precocious teenage boy who has flunked out of several prep schools. Salinger did not want the reader to think that the manuscript had passed through the hands of professional editors at some publisher's office. We may be reading a published novel, but Salinger evidently intended to create the illusion that we are reading a long memoir in some kind of manuscript format.