Can I get source access to the NT kernel on Windows? Nope. That's Microsoft's secret and novel invention that is supposedly the best OS in the world, so they have to keep it on lockdown. </s>
Just being restrictive of what computers can run your OS does not make it closed. When has Microha$h ever supported FOSS? Think about it for a second.
OS X is specifically designed not to work non-Apple hardware, even hardware that is perfectly capable of running it.
Parts of it are open source, mostly because those parts were an extention of existing open source software.
"OSX" is not free in any sense of the word.
Microsoft are still a far cry from the likes of GNU/Linux in terms of openness, but almost nobody would describe the Mac platform as "more open" than Windows.
In Windows: "Metro" apps didn't exist before Windows 8, and normal apps don't have any origin restrictions.
In OSX: Gatekeeper is easily bypassed by the user for specific apps or even turned off entirely.
Which means Steam and all its titles would have to be launched by explicitly navigating to the classic desktop. Which is beyond Apple's simply providing a parallel way to get software onto OSX.
If Apple didn't allow icons in the Dock, unless the app came from the Mac App Store, I'm sure you'd see a similar reaction from third-party platform creators like Valve.
If Steam can still trivially get its third-party-bought programs onto the metro-styled start screen, the popular reporting on that situation was off.
And I can't even find a story where Gabe himself actually expresses an opinion on, or concern about, the MS App Store. His comments seem to entirely revolve around Metro being bad for desktop usability and Windows 8 not selling. So the popular reporting that Gabe was specifically upset about the store (via it's limitations) seems to have all been bullshit/editorial.
Upon digging deeper, it seems his concerns about Windows 8 aren't materially different than his comments about Vista (via complaints about DX10 being Vista-only, as Vista wasn't selling).
All mine are, by default. Valve is concerned about the competition from the Windows app store, not the UI revamp.
In my poking around on this, I saw plenty of assertions that this was all about the app store, but nothing is ever attributed to Newell except his lament that the usability is off and the sales aren't there.
But the 'doom and gloom', from Gabe's mouth, doesn't seem to be any more extreme than his stated position on DX10/Vista. As far as I can find, the press are the ones taking his comments to extremes, reading into them about app stores, etc.
I think they have to move quickly though - the steam-sale model is getting copied by other PC download services and they stand the chance of getting lost in the crowd.