Gu Xuanye is probably afraid that if she knew the truth, she wouldn't be able to accept this relationship...
Mo Qiansu didn't want to tell him that she actually already knew. She'd rather bury this secret in her heart forever than burst this bubble and starkly confront the moral decay of their relationship.
At least this way, future encounters wouldn't be too awkward...
Mo Qiansu raised her head, and finally met his gaze again, slowly began to speak, "I did see your grandfather today, but you're mistaken, he didn't threaten me..."
"If he didn't threaten you, why on earth would you suddenly bring up breaking up with me?!" Gu Xuanye interrupted her urgently.
In his heart, he felt even more angry at his grandfather's actions.