When we are talking about writing in games, we are talking about things like story, quests, dialogue, narrative, character development. When we are talking about game mechanics, we are talking about designing games and sitting in Excel spreadsheets doing math magic all day long.
Neil Druckmann and Steve Gaynor briefly went over this topic in a Tone Control podcast. Check out Episode 2, as they spend a segment talking about how to make it as a writer in games: https://www.idlethumbs.net/tonecontrol