lol.. 'progressive death metal' why're gojira progressive? their songs don't really progress. music labeling is so stupid
He's a metal fan, of course he's going to know the Wikipedia-correct genre (I tend to label my genres by the Wiki default). At some point, you amass so much of an all-encompassing genre (like metal, rock, etc.) that you decide "____ doesn't sound like ____; I need some sub-genres".
Plus, going by sub-genre is a good way to learn about bands that sound like the bands you enjoy. If you say you like rock, you get a painful barrage of anything from the Beatles to Nickelback to Fang Island. But if you tell me "I like avant-garde metal", I can suggest a few specific bands for you.