As Luk Siu Fung told Fa Mun Lau, it was extremely difficult to get Sai Mun Chui Sheut motivated into taking action to help others unless Sai Mun himself had a reason to be interested in getting involved. Considering that Sai Mun Chui Sheut (at this point, especially) cared for nothing other than the sword, this was next to impossible.
Luk Siu Fung ultimately maneuvered Sai Mun Chui Sheut to help him in the Golden Roc Kingdom's lost treasure case by using the fact that Sai Mun Chui Sheut hated having his actions predicted by others, but I don't think it's as simple as that. I doubt Sai Mun Chui Sheut would go through all that trouble and let himself get manipulated just because Luk Siu Fung and those two "know it all" guys in the cave had apparently predicted that he would not, under any circumstances, help Luk Siu Fung.
Consider these reasons:
1. It just so happened that Sai Mun Chui Sheut's home of Ten Thousand Plum Blossoms Manor was located in San Sai, which coincidentally, was where Yim Teet San's (Yim Lap Boon) Pearl and Jewel Pavilion was located. It stands to reason that Sai Mun Chui Sheut did *not* want the likes of the Green Shirts Pavilion gang causing trouble around his territory. Even if Luk Siu Fung had not asked him, Sai Mun Chui Sheut probably would have taken down the Green Shirts Pavilion on his own anyway after he learned that they were operating in his backyard.
2. At the time, Sai Mun Chui Sheut lacked any really good competition. His most recent opponent, Hung To, had fallen to him in just one stroke. Luk Siu Fung's descriptions of Yim Lap Boon and Ping Dook Hok (Dook Goo 1 Hok) must have intrigued Sai Mun Chui Sheut. Each would provide much better competition than the likes of Hung To and would better prepare him for his eventual duel with Yip Goo Sing.
These two reasons, in addition to the reason supplied by Luk Siu Fung, probably sealed the deal for Sai Mun Chui Sheut getting involved in this case.