Gao Yang was trained, knew how to quickly interrogate after capturing someone on the battlefield, but he was just a mercenary and surely never thought about learning how to conduct professional and brutal interrogations.
Of course, even if Gao Yang wanted to learn, there was no one to teach him back then. Number Thirteen and Yake mastered some of those skills, but to be honest, they weren't professional.
An assassin and someone who's in intelligence yet works in the field, they all understood but weren't specialized in interrogation. At least compared to those who mastered everything, not just the basics, like the Black Demon, they weren't as professional.
