There are a number of numerical inconsistencies in the book and this appears to be one of them. A 5 minute all-out effort will be done at a power level much greater than 100% FTP, since A&G's definition of FTP is "the highest power that a rider can maintain in quasi-steady state for approximately one hour without fatiguing."

The protocol is fine if you do the 5 min at an all-out effort. It will give you more data points for generating a Critical Power curve. Pacing for both efforts can be challenging for the novice, but gets better with experience. I've developed a feel for the efforts with my breathing rate and tension in the legs.