That is an apples and oranges comparison. While you speak of "the best English writers I know are native speakers of English", the post you compared to was "the best writer among the students at her school tend to be people from other countries who have undergone rigorous language training". (emphasis added)

While you speak of Hemingway and Steinbeck, the college instructor talks about students, one of whom has been extensively schooled in proper English and probably took the instruction seriously. This says nothing about the education of her other students and it doesn't hit on accents and understanding how they speak, it's about writing.

How many of us end our sentences with a preposition? That's a rhetorical question.

opinion, $.02