I think the exercise is more about being clear and consistent in what you expect from a dog in response to a particular command. If a "sit" is met with the dog rocking back on his butt and then popping up again, and that's rewarded, then there will be no actual SIT offered when the owner requests a "sit."
I see this all the time in agility with people who don't or won't teach start-line stays to their dogs. They sit the dog and walk to a lead-out and tell the dog to "stay"; the dog gets up and starts to the first obstacle and the handler begins the course run. Let's imagine what "stay" means to that dog, eh? Pretty much "when I get X feet away from you, start your course, and I'll try to keep up with you and direct you."
I don't think McConnell means you can't use the same word for different things. As Sabine said, dogs are masters of nuance and can infer everything they need to know from inflection, emotion, and body language, no matter what the word is. In fact, going to agility again, some of the best runs ever turned in have been silent runs: the dog was cued only with the handler's body language, and ran a lot better than when the handler jibber-jabbered his/her way through the course. Which shows me that dogs don't depend on our words, else all those dogs would still be sitting at the start lines of the courses, and hesitating at the obstacle discrimination points on the course.
More powerful than any word of praise, IMO, is an eye-to-eye look and a chest skritch for the dog. The affection and esteem is communicated without words.