What you call evidence, it only counts as evidence if people believe it; otherwise, it's no different from trash.
What Qing Yu needed to do was to turn the evidence in Zhang Yuanchu's hands into trash.
If Qing Yu were a nameless soldier, even just the Director of the Northern Zhou's Qintian Observatory, people would believe Zhang Yuanchu's words and identify Qing Yu as Gong Ziyu.
But now Qing Yu is the Sect Leader of the Zhenwu Sect, on par with Zhang Yuanchu as a Divinity Realm warrior. With equal status and strength, even slightly higher status, Zhang Yuanchu can't directly pin the label of Gong Ziyu on Qing Yu's head.
Using something written by Qing Yu's enemies to testify against him is as ridiculous and absurd as using a sword from the previous dynasty to execute an official of the current dynasty.
