If you read it again, you'll realize he recommends 20 minutes of intense cardio, with an RPE of 7-9 (or something like that.) That's pretty tough to do consistently for 20 min. (20 minutes of medium-fast runnung will give you a better workout that 20 minutes of slow walking.)
Also, on the other days you're lifting pretty heavy weights, and that also burns calories, along with raising your lean muscle mass and metabolism.