Admittedly, it was a bit strange for Duan Xiaomo to say this, since he was a member of the Celestial Sect, and Lu Chengfeng was a special guest of the Celestial Sect.
However, since Duan Xiaomo said that he was teaching him a lesson on Ling Chuxi's behalf, it was acceptable. This was because Ling Chuxi had been teaching him martial arts, and he had acknowledged Ling Chuxi as his master. Helping Ling Chuxi meant helping the Golden Pill Sect. There wasn't any problem with it logically, but the legitimate heir and young master of the Celestial Sect helping the Golden Pill Sect teach someone from the Celestial Sect a lesson was really quite comedic. To Lu Chengfeng, such an act greatly tarnished his reputation.
Duan Xiaomo did not consider that, and did not care either. After he had finished speaking, he started walking over to Lu Chengfeng.