There are a lot of Steam gamers with 5 games in their library who log on once a month. There are a few Steam gamers with 5000 games in their library who are permanently logged in. There's folks who play one game obsessively, and folks who tinker around with many games.
I'm willing to bet that the 3% are the kind of people who buy a lot of games.
I'd love to see that "what percentage of games have been bought by people on which platform?" metric. I think it'd be a lot more than 3% on Linux, even if you count Steam Deck as a separate platform.