It seems like sbierwagen's linked article makes this a bit clearer than the op.

I think that the fear he has of Windows 8 is not that Valve will have competition or that steam will lose market share. it already is fighting similar offerings from a few different angles. He seems concerned that windows 8 represents a step back from the open platform model of windows. That it signals an attempt from Microsoft to build their own walled garden to counteract Apple's. This would inevitably be bad for third parties who make their living in the Microsoft ecosystem.

Linux then solves the problem because the platform is open. Not in terms of cost but of access