For example, I have Steam installed on my Macbook pro and I occasionally play a single very simple game there. Does that make me a macOS gamer? of course not. The vast majority of games I want to play don't work on macOS.
I suspect that most of those 5% are just Linux users who have steam installed and play a small amount of games. Some probably just installed it to check what's available and don't play anything.
Everyone I know who is a "serious" gamer, as in exited about upcoming releases of AAA games is using Windows.