Su Xin attacking the vassal sects of Du Gu Family on their own territory was essentially a slap to the face of the Du Gu Family - and Su Xin did so intentionally.
Currently, it would indeed be difficult for Su Xin to strike directly at the Du Gu Family. When he destroyed the Qingcheng Sword Sect, he was able to utilize the power of the Imperial Court and the Heavenly Court of the Netherworld, whereas when he crushed the Su Family, he primarily tapped into the strength of the branch members of the Su Family.
Now, when faced with the Du Gu Family, Su Xin couldn't use the power of either of those sources.
Moreover, due to the highly sensitive location and status of the Du Gu Family, even if Su Xin could muster an adequate force, Ji Haodian would never allow him to launch an attack.
