I didn't read the book, but I read some of Mike's articles back when he was still competing.
Mike belived that only a small amount of extra calories above and beyond ones maintainance requirements are needed so support maximum muscle growth.
Right or wrong, his reasoning does make some logical sense.
Think about it like this; If you could succeed in gaining one single ounce of lean muscle mass a day, every day for a year, you would gain 23 ppounds of muscle. That's a pretty good gain for a single year!
Now consider this; How many calories do you need to eat to gain one ounce? I think there's about 400 calories in an ounce of fat. An ounce of muscle is probably close to the same. Do you need to consume thousands of extra calories daily in order to convert just 400 of them into new muscle? Mike's answer was no. However, there are lots of bodybuilders consuming 6,000 plus calorie per day diets that would disagree!