If Fang Mu were to truly let Clan Zhenyan go, the clan would instead be filled with unease.
Because Clan Zhenyan really had no path left to take.
As soon as Clan Zhenyan showed their face they would instantly be targeted, encircled by powerful enemies.
Even if Clan Zhenyan told those powerful enemies that they no longer had the ancient books, the enemies wouldn't believe them. They would surely capture Clan Zhenyan and extract information through intensive torture.
Moreover, Clan Zhenyan was still unclear about the force that the young man in front of them wielded.
If asked who took away the ancient book, they wouldn't be able to give a clear answer either.
"In return for Young Master Fang's query, Clan Zhenyan has nowhere else to go."
"We are willing to be obedient and serve you to the best of our abilities!"
"Clan Zhenyan is willing to venerate and worship you like a deity, we hope that you can graciously accept Clan Zhenyan as part of your chosen clan!"