Considering the paucity of games written for OS X I think its a stretch to see any number of games running on Linux. Even on OS X far too many are ports or wine type encapsulation and the OS X base does tend to spend more than what is normally attributed to Linux users.
On Steam now, there are 289 Linux capable games, versus 809 for OS X, and thousands more for Windows. So they have a lot of ground to make up
There is nothing wrong with "Wine type encapsulation" ports, especially when we're talking about full screen games that don't need to interact with any sort of desktop metaphor. If it works, the user just doesn't care (or even notice!)
The difference here is that Valve is strongly pushing for Linux, and helping out developers to follow their path. That did not happen with OSX. And they know that serious gamers are not OSX users. The people who play the most are on Windows currently, and even if they convince 1% of Windows users to switch to Linux at some point, it will make a significant impact on the Linux gaming market.
OS X also still have large problems with graphics drivers while on Linux the drivers have gotten way better the last couple of years.
When looking at the tech support forum for Europa Universalis 4 I noticed Mac users had much more performance problems than either Windows or Linux users.
I think they mean Linux will be successful in the gaming space in the same fashion it succeeded in the mobile space. The Steam Box will run Linux, no user will ever know about it, and it will leverage the openness of the OS, coupled with the already existing Steam user base to lure developers.
I think this is a game changer for the console game development industry, and for the console market. In the end, however, it'll be a lot like Android: quiet success.
On Steam now, there are 289 Linux capable games, versus 809 for OS X, and thousands more for Windows. So they have a lot of ground to make up