To back up Yang Yuqi, he simply lied that he was a personal disciple of the Shushan Sword Sect's sect master, because this identity was intimidating enough.
Given his relationship with the sect master, there wasn't really any major problem with saying this, and if the sect master knew, he would certainly back him up.
Furthermore, the sect master was actually dying to take Meng Fan as his personal disciple, but was just too embarrassed to snatch a disciple from Lin Bianyun.
Chapter 319