I wouldn't be surprised if a survey actually overestimated the number of Linux users. Using Linux for gaming is a choice and usually a more difficult path than gaming on Windows. So, Linux users are likely move vocal about their use of Linux, for various reasons: (1) it's often a principled choice; (2) by showing that a significant chunk uses Linux, more gaming studios will target Linux.
tl;dr: it would be nice if Valve released actual stats, but I think the survey is a good ballpark figure. In other words, the market is pretty much non-existent.