Due to this not being a head-on collision between two horses, Zhao Haiping could have avoided falling off the horse by adjusting his posture to maintain balance and by clamping his legs tightly around the horse's belly.
But at the moment when the two horses crossed paths, he deliberately relaxed his legs and jumped down from Chi Ji with the momentum.
The reason was simple; the goal of this instance wasn't just to assassinate Gao Yi and end it there.
What followed was also crucial.
"To gallop straight into the midst of the enemy forces and stab the target, to chop off his head and return, no generals can stop him, the siege is thus broken."
There were two key points to accomplish here: one was to "chop off his head and return," and the other was "no generals can stop him."
Without completing these two points, the Trial Illusion wouldn't be considered passed.