It's very apparent that XCOM (the game referenced in the article), even in the Steam version was very much designed for touch screen use. The game requires no continuous input or precise input that a keyboard + mouse would provide, and as such is ideal for tablet devices.

The same can't be said for most games. It's not really a case of steam directly making their library available to mobile audiences, it's the lack of appropriately designed content.

IMO most virtual joypad style games on mobile devices offer a terrible user experience, and the majority of touch input focussed games wouldn't be as successful for a PC gamer audience.