The case of the self-refuting linguist

John McWhorter, in his book "Our Magnificent Bastard Tongue: The Untold Story of English", has an interesting contradiction in thought that doesn't seem to have occured to him. It is connected to the Sapir-Whorf hypothesis, which McWhorter starts to poo-poo on page 138:

The idea that grammar is thought became influential from the writings of Edward Sapir. We met him in the previous chapter venturing that English speakers came to find nuance irritating. Even that point had hints of the language-is-thought persuasion -- supposedly the erosion of various aspects of English grammar was due to some psychological leaning in its speakers. But Sapir ventured only passing speculations in this vein.

It was Sapir's student Benjamin Lee Whorf who picked up the ball and ran with it, in the 1930s, publishing several pieces on the subject which served as its foundational texts. The hypothesis is known, therefore, as the Sapir-Whorf hypothesis.

The hypothesis has also failed. Repeatedly and conclusively.

Earlier in the book, he devotes a large amount of space arguing for the usage of the singular pronoun "they" instead of the singular pronoun "he" when refering to an person of unspecified sex, basically on grounds of sexism (p. 66; italics in original):

My own books are full of resorts to he, which I find sexist, occasional dutiful shes, which stike me as injecting a stray note of PC irrelevance into what I am discussing, or he or she, which I find clumsy and clinical -- for the simple reason that I was required to knuckle under. At best I can wrangle an exceptions and get in a singular they or their once or twice a book.

But why do contemporary writers consider the singular, indeterminate sex "he" to be sexist? It's the Sapir-Whorf hypothesis: eliminate sexist grammar from English and sexism will magically disappear!