The 2nd and 3rd editions of Training and Racing with a Power Meter have the same table of expected adaptations, and no column for sweet spot:
“Table 3.2 lists the primary physiological adaptations expected to result from training at each level, although these will obviously be influenced by factors such as the initial fitness of the individual, the duration of each workout, the time taken between each interval effort, and other factors.”
The guy that coined the term “sweet spot” is Frank Overton of FasCat Coaching. Going back to duration of workout, his sweet spot training plans will have you doing a lot of zone2 endurance work. The intro of some FasCat podcasts is a Frank soundbite “I was always an advocate that you could accomplish more with sweet spot training than riding in zone2 alone.” Emphasis more than “riding in zone2 alone” (the long slow distance or TR Traditional Base 1 concept).
Point being, there is value doing Zone2 Endurance work despite what you see in Coggan’s table. Maybe Inigo San Millan needs to do his version of expected adaptations.