So, this would seem to be fairly easy to test empirically. Get a reasonably object measurement of the quality of writing, and use it on something where you know if it's true: 1) court testimony which we know (from outside evidence) is either true or not true 2) scientific papers which we know to have been reproducible, or not 3) stock pundits predictions about the future of some company or other, which we know with h…
There are objective features of writing, but quality is subjective.
Of course, as to the thesis of the essay, it is both trivial and uninteresting that people, including PG, tend to have views of the correctness of an idea and the quality of the presentation that are correlated.
It is interesting that PG thinks that this is anything more than a cognitive bias to be cautious about, though.