We should also be aware this is a memoir, so virtually every dialog in it is a best a paraphrase, because who can really remember exactly what was said in a conversation 30 years hence.
And a lot of them are obviously embellished for humor. Going by this book, these two were the funniest guys in showbiz....
Of course, while the actual "quotes" might be paraphrased for the sake of fun, I don't doubt Bob Justman's sense of humor. Enough of his internal memos have been published to indicate that even during production he had a dry wit which was legendary. The one which begins with "I am in receipt of a memo from some character using the pseudonym D.C. Fontana" (and then subsequently refering to the name D.C. Fontanta in quotation marks all the way through the rest of the memo) is one that always raises a chuckle from me.
I don't doubt that those guys had a genuine sense of fun and interplay between them. But I agree, the discussions in the book have obviously been embelished for the benefit of making the book entertaining. Nothing wrong with that.