The premise that Yeung Hong learned martial arts from Yau Chui Gei is problematic in several aspects:
1. How was it explained to the young Yeung Hong why this Cheun Jen Sect Taoist began training him in Cheun Jen Sect martial arts from the time that Yeung Hong was a young boy? Yeung Hong *must* have been curious as to why he was getting this martial arts instruction (he didn't know anything about his scheduled duel with Gwok Jing or that he had to get revenge for a father he didn't even know existed). The boy would've started asking questions (of Yau Chui Gei, of his mother, and of Yeun Nan Hung Lit...how did they prevent Yeung Hong from letting the cat out of the bag on that one?) that his elders couldn't answer.
2. Didn't Yeun Nan Hung Lit ever wonder where his adopted son's martial arts skills were coming from? Yeung Hong's martial arts weren't the techniques practiced by Yeun Nan Hung Lit's mercenary thugs.
There's no way that Yeung Hong could've learned the Cheun Jen Sect's martial arts for all those years without other people finding out about it.