The first character that was known to have been castrated was Tin Ba Guong.It was only later after DFBB's appearance that explained the need for castration to learn the sunflower scripture.Then next revelation was that Ngok Pat Kuan and Lam Ping Gi had done that to themselves.
We all know that TBG never came across the pi xie sword style(Lin Yuan Tu's robe) but could there have been some plot anticipation from the readers that Tin Ba Guong might bump into the pi xie sword style sooner or later and have learned it since he had the "qualifications"?