I actually did find "idiomatic" when I looked it up, but I honestly still didn't quite grasp it from the cambridge dictionary. Thanks for explaining it in a way I understand.
In programming Idiomatic is used to reference a programming language’s “best practices” and “style guide”. Obviously programming languages can solve problems in many different ways, but they often develop a “correct way” that matches their design or the personality of their influential community members. Following this advice is Ideomatic. Next time your coworker has their style wrong you can say “I don’t think this is Idiomatic” :D