Based on Gu Xijiu's thoughts, she felt she should explain it to Long Siye now as she thought he would not hurt Celestial Master Zuo as long as they explain it clearly. She believed Long Siye would understand.
After all, Long Siye was an honest person and his reputation was actually better than Di Fuyi's.
Both of them were heaven's gift disciples; he would not hurt Di Fuyi at this point. Perhaps, he could even help out when they exchanged their bodies back if he knew the truth.
However, no matter how Gu Xijiu tried to persuade him, Di Fuyi refused to surrender.
Gu Xijiu had no choice so she forced him to select the second option, which meant that he must tell Long Siye the truth after they successfully exchanged bodies.
Di Fuyi finally picked the second option as Gu Xijiu threatened that she would tell Long Siye the truth now if Di Fuyi refused to make a promise.
Therefore, both of them were happy now.