Could be that the real picture is more complicated than how it's presented in the pilot training manual. Maybe they just put the N2 control as a general statement. But, for example, the previous plane I flew had hydro-mechanical N2 control up until 80% N1, and above that an electronic N1 control kicked in instead. And if it failed or was turned off (as happened to me once) the HM N2 control was active for the full thrust range.