Even though Feng Jiuge's cultivation had regressed for two years, he had managed to temper his soul and spirit well within this period.
From his lofty position as the genius of the Inferno Phoenix Clan, he had fallen to the bottom and experienced the warmth and coldness of the world. These experiences were invaluable in building up who he was, and it had made his soul more resilient than ever.
Due to this, the God King Breakthrough Pill was particularly effective on him, allowing him to successfully make a breakthrough.
Had it been anyone else, it would have been difficult for them to make a breakthrough so successfully.
Other than that, Feng Jiuge's body had been thoroughly cleansed by Zhang Xuan's zhenqi while he was getting rid of the gray congregation of energy. That had played a huge part in building up Feng Jiuge's foundation too.