Jamie Purviance is without a doubt one of the great experts on using kettle grills. He says the same thing in his other book "Charcoal Grilling". He prefers to control the temps with a kettle with the top vent even though on the very next page is describes how the WSM is controlled in exactly the opposite manner. As stated above, it all about airflow and how you control it. Personally, when I do low and slow on the kettle I set in mainly through the bottom vents, but I also adjust using the top vent as necessary, in particular if I have overshot my target temp. When going high temp direct or indirect grilling, I always leave the bottom and top vents 100% open. I adjust to the temps I need by other means such as building multi-zone fires, adjusting the amount and placement of coals, as well as the decision to use lump vs. briquettes.
WSM, 99 & 09 Performers, 2-OTG, SJG mini WSM, Summit 420, Q100, Akorn, and Schwenkgrill