Steam reports 3.4% for linux and 2% for macos (last montg. This month numbers are probably hit by systematic measurement errors).

https://store.steampowered.com/hwsurvey/

Also, the 16% “unknown” in the graph you linked to implies huge error bars on macos vs linux!

Steam stats aren't particularly meaningful in this context, because MacOS is largely unusable for gaming due to not being able to run most games.

Valve themselves have given up on supporting their games on Mac, likely due to the total lack of backwards compatibility.

That's desktop market share. I'm sure the comment you're replying to means overall usage including mobile and server, where Linux is far and away the leader.

Why would anyone compare market share of a desktop OS and include server / mobile / embedded?

Because they want to make Linux sound good I guess?