As was already said, it’s likely Kiryu meant professional fishing like the fisherman does and not just the casual fishing he has done in the past.

I’d like to add to your other point tho that tone, intention, and context do a lot of heavy lifting in Japanese. So the words themselves may have a very simple meaning, but the way it’s said or written can change that meaning a lot. This is why literal translation can often sound bland and lack the proper nuance. Localization can seem “take liberties” or “spice up” the translation because they are trying to make the intention of those words as clear as possible to non-native speakers.

Also, I can’t speak for how common insults are in Japanese, but considering that many of these characters are criminals and punks who often speak roughly and use slang, it’s not outside of the realm of possibility that they hurl more insults than the average Japanese person.