I like the creative number interpretation that's going on. Sure Microsoft has lost some ground recently and Windows Phone has a hard time on the market (which I personally find rather sad because after having used iOS, Android and Windows Phone I can definitely say I like WP the most) there is absolutely no data to indicate that there is any large scale migration towards Linux going on. In spite of all the privacy issues, Windows 10 has, a mere month after it's release, three times the desktop market share that Linux has. Just let that sink in for a spell.
If anything, casual users tend to use their tablets or smartphones intead of PCs these days, but none of those are likely to actually care for games that much. Aside Angry Birds and Candy Crush Saga, obviously.
So perhaps Steam OS will shake things up a bit, but honestly, it's not the opinion of Brandon holding Linux gaming back, it's the lackluster support of certain companies regarding drivers that will REALLY be a problem (or already is one) - even for Valve and Steam OS. Can't quite shake that much up if half the graphics card market decides that they don't want to play ball.
But yeah, whatever.
In the other hand it is good to keep in mind that Linux is brand new in gaming business and it will definitely take some time to catch up. Microsoft has been there around 30 years already . Naturally Microsoft will do everything to keep Windows as the PC gaming OS. I don't expect Linux gaming to be big still for many years. Vulkan drivers and development tools has to mature and the developers requires much more experience - I guess most them has heavy MS DOS/Windows/DX background.
I personally think Linux/SteamOS is very good addition for PC gaming; more choices and more control for PC gamers.
Microsoft uses hundreds of millions to marketing Windows and they have all the OEM contracts. Linux has the community, so it requires a bit of pushing