When Hua Junzi noticed Su Weidong and his group, he was filled with confidence.
Dealing with such minor characters didn't worry him. He intended to toy with Xiao Jie at his leisure.
Considering how far Mo Huasong was in Capital City, it would take him quite some time to fly back even if he wanted to.
By the time Mo Huasong returned to Mo Manor, who knows where Hua Junzi would have taken Xiao Jie, or how many times she would have been toyed with?
In reality, it wasn't Mo Huasong's negligence, but Xiao Jie's willfulness that was to blame.
With the Nine Dragons Trapping Kill Formation left by Mo Huasong in Mo Manor, Hua Junzi would have no way to touch Xiao Jie.
Furthermore, Su Weidong was powerful and had strong defensive abilities, not to mention immune to poison, so he wouldn't be in danger when facing Hua Junzi either.
That was why Mo Huasong intentionally left Xiao Jie back while he went to Capital City, hoping to lure Hua Junzi out.