a) how would he know them? having begun his study of Chinese during the Cultural Revolution and unable to visit China at all until he was 27
b) he'd like to avoid the possibility of people trying these words out on any visiting Chinese they happened to meet, to demonstrate their fluency in the colloquial vernacular.
"Suppose there was a book published in America about the Japanese language with all these kinds of words in Japanese plainly laid out, and some smirking American with a smattering of Japanese came and used them in conversation-- how would you feel? First off, no one would pat the American on the head with a 'My, how well you know Japanese.' They'd have deep doubts about his character and wonder where on earth he'd learned the words in the first place."
Suppose, sensei? Past praying for, I'd say, if you asked me.