There's a bit of a selection bias using the steam hardware survey - I'd bet that a greater proportion of windows gamers than linux gamers are using steam.
I suspect there are also a lot of Linux users who keep around Windows to run Steam and games in general, and who happened to be in Windows when they ran that survey.
Each time the survey appears you can elect whether or not to participate. If you are a Linux user currently dropped into Windows to whom this issue of OS identity matters, you can opt out.
True, but that still then requires that they then reboot into Linux for the sole purpose of completing the survey, and it assumes that they can install the Linux version of Steam on the distro they are running so that they can take part in the survey in the first place.