The author actually discusses potential counterevidence: Cotrell’s few superior articles. She dismisses this evidence by saying that the articles must have been plagiarized, but she doesn’t ignore the existence of counterevidence.

The author doesn’t generalize from atypical occurrences. Instead, she draws a specific conclusion about Cotrell based on all of Cotrell’s articles— the superior ones and the average ones.

The author seeks to establish that Cotrell can only write average quality articles. In order to do so, she presupposes that he can only write average quality articles.

The author never mentions or relies on the judgement of any experts.

The author does infer limits on Cotrell’s writing ability. But she does so based on his average work and the claim that his superior work was plagiarized, not based on “a few isolated lapses in performance.”