Why do they have a human doing this? We have computers now, and solving this kind of cipher is a Solved Problem. What this guy spends days on, a computer could do in milliseconds.
I guess the problem is domain specific knowledge, user smcl did write something about that below.
Law Enforcement is a very tight knit community. They trust each other and that trust builds over the years. They are almost like a family. And apparently, this is their go to guy for gang codes.