Perhaps it's because Reiger's methods are excessively violent during the Shikigami subjugation rituals of the "Ten Shadow Techniques," his subjugation of each Shikigami could be deemed as achieving the pinnacle of perfection.
Subjugation means to conquer, and with Reiger's strength, dealing with the Shikigami of the "Ten Shadow Techniques" meant that the process was entirely one-sided, even for the final Shikigami.
Under such circumstances, Reiger suddenly realized that while summoning Shikigami of the "Ten Shadow Techniques," he no longer needed hand shadows.
The Shikigami from the "Ten Shadow Techniques" were extremely obedient to him, and his control over them was exceptionally smooth, with no obstacles, even easier than summoning creatures through Summoning Magic.
This aspect also demonstrates the adaptability of sorcery, unlike magic where the effects are completely fixed.
