On top of this, I would bet that they have so many Linux contributors precisely because no-one releases AAA games to Linux, so their sample size is dispro

On top of this, I would bet that they have so many Linux contributors precisely because no-one releases AAA games to Linux, so their sample size is disproportionately represented by Linux users. That's the point tho - if you're an indie developer it might be worth your time to develop a cross platform game even tho not that many people use linux & mac compared to windows (since you care about people who buy your game not people who run that particular OS).