He saw Pei Xuanjing as a young tiger, not because he underestimated Pei Xuanjing's strength. Being able to kill the Lord of Shenxiao Gate, he would never take him lightly.
When he called him 'young', he meant that his rise was too brief, and he didn't have the backing of any major force.
So even though he now has a tremendous power, he lacks in many other aspects, and all these deficiencies require time to rectify.
Of course, all of this is based on the assumption under which he was unaware that Pei Xuanjing had arranged for Yu Zhenzi to take over the forces left by the Lord of Shenxiao Gate. If he knew this, perhaps Zhao Baiyang wouldn't have said such a thing.
"I see!" Yu Yiyao's expression grew gravely serious.
She didn't have as many considerations as Zhao Baiyang. Hearng her master's words, she simply compared and deduced that Pei Xuanjing, who was able to kill the Lord of Shenxiao Gate, was naturally stronger than the former.