Shangguan Yanqing and other three were attacking Su Xin, but they didn't take the opportunity to ambush Xie Zhiyan, who was absorbing the Sword Pill. It wasn't that they didn't want to, but rather they dared not.
Jianghu has its own rules, and righteous sects like theirs also have their rules.
When facing forces inferior to them, those rules could be dismissed, but they couldn't ignore them when dealing with the Yi Sword Sect, one of five sword-holding sects backing Xie Zhiyan.
If they ambushed Xie Zhiyan and actually killed her, it would be fine. But if she survived, it would result in a monumental feud between the two sects.
Cao Zheng'an didn't mind in the least. Although he wasn't a righteous man, he wasn't accustomed to being hunted down by others like demon Tao disciples. He didn't want the latter half of his life to be spent evading the Yi Sword Sect on daily basis.
Therefore, their immediate target was Su Xin. They planned to deal with him first and then Xie Zhiyan.