>>cookie+(OP)
Somers, too, implicitly criticizes "using fancy language where fancy language isn’t called for"—"fustian" isn't a compliment when used of prose. The adjectives paired with it in Wiktionary tell the story: "Dutch fustian", "wretched fustian", "mere fustian", "genteel fustian which lacks either poetic resonance or demotic realism".
But he doesn't really give any writing advice in his essay. He doesn't recommend that you write fustian or that you write like Hemingway or indeed that you write at all; instead, he recommends that you read the dictionary because it will be fun. So, if we're talking about writing advice, we need to look at McPhee's essay, not Somers's.