When "John The Mod," now "Johnny Jekyll," seems to make a faux pas by mentioning death metal, it may be an intentional attempt at a joke. He never refers to himself as being Black Metal or Death Metal during the duration of the film, nor does he ever state he's an extremist. He's only described as "Norwegian Black Metal" by other characters.
John The Mod is labeled as Norwegian black metal, yet mentions he will do a "Death Metal" version of the song "Smelly Cat" from Friends (1994) at some point when talking to Sean.