The Divine Light has already proven through action that the power of Divine Seal Fragments cannot defeat the Master of Divine Seal. Since holding onto them only turns them into a weakness, Xuanming could only discard them all.
From the position of those challenging Xuanming, this is indeed a huge piece of good news. I also possess the power of Divine Seal Fragments, yet it has scarcely improved my own combat power; although I can enhance this through the blessing of Little Bowl, it merely results in a "when the enemy is killed, they die" dark humor effect. While it's very effective against enemies with an Undying Body and will certainly be a trump card for future battles with Ming Zhuo, it doesn't show its power in ordinary combat.
