Do you have a link to a quotation? It's not exactly a rare assumption, why would a company predicate almost all of their business on a platform essentially owned by another company? It makes a lot of sense to use Linux, and frankly I hope it continues. The less people have to deal with an incompetent company like Microsoft, the better.
Check out the Steam Dev Days videos on Linux Development and moving to OpenGL. The repeated refrain from Valve is that you don't have to choose between linux and windows, you can choose both. I believe it was Ryan Gordon who made the point that abandoning windows means abandoning 95% of their current market, especially when China overwhelmingly runs Windows XP. The goal isn't to get people to stop supporting windows and start supporting Linux, it's to get people to support
steam, no matter where it is. It's a benefit for all developers.
Valve's vision isn't "let's abandon the platform where we make all our money and start over," it's "let's get our games running on
everything." Their talk about building towards the Steam runtime library rather than any OS proprietary libraries really drives the point home, the very benefit they proclaim is that by doing that, your game will run on anything.
EDIT: Some videos:
Beginning Linux development:
https://www.youtube.com/watch?v=Sd8ie5R4CVE&list=PLckFgM6dUP2hc4iy-IdKFtqR9TeZWMPjm&index=6
Moving to openGL:
https://www.youtube.com/watch?v=45O7WTc6k2Y&index=9&list=PLckFgM6dUP2hc4iy-IdKFtqR9TeZWMPjm
United we win:
https://www.youtube.com/watch?v=4cjfpIpy6ZM&index=11&list=PLckFgM6dUP2hc4iy-IdKFtqR9TeZWMPjm
Debugging with linux:
https://www.youtube.com/watch?v=xTmAknUbpB0&list=PLckFgM6dUP2hc4iy-IdKFtqR9TeZWMPjm&index=12
Optimizing Linux games for AMD using GPU Perf Studios:
https://www.youtube.com/watch?v=biUffE9BB0I&list=PLckFgM6dUP2hc4iy-IdKFtqR9TeZWMPjm&index=13
SDL 2.0 overview:
https://www.youtube.com/watch?v=MeMPCSqQ-34&list=PLckFgM6dUP2hc4iy-IdKFtqR9TeZWMPjm&index=23
SDL 2.0 in particular is a huge backbone of Valve's cross-platform development strategy. To show it off, they showed a single codebase running on Windows, Mac, Linux, and iOS, all at once, without a single change being made.
a specific sentiment from Ryan Gordon about "linux:" We shouldn't think of development like "developing with linux" or "developing for windows" or "developing for playstation." Instead, think of your development as "developing exclusively for windows" and "developing not-exclusively for windows" where the latter option is also inclusive of windows.
That was the whole point of Dev Days (the dev portion, at least).