What I think he means is that separating your workouts into zone 2 and 5 is "optimal" in terms of anaerobic contribution. At zone 2 your anaerobic contribution

What I think he means is that separating your workouts into zone 2 and 5 is "optimal" in terms of anaerobic contribution. At zone 2 your anaerobic contribution is low, while at zone 5 it is high. Its best to think of these energy systems as a sliding scale, and his model is based on the theory that zone 2 is the optimal window on this scale for improving metabolic health. This doesn't mean that zone 3 and 4 are useless. Most endurance athletes do most of their high intensity in these zones because the stress of zone 5 is not manageable over time.