Did you read the article? Gabe said the SteamBox is running Linux so everything will just be OpenGL etc etc
I would be very surprised if Valve can drum up the hardware numbers to sell it at $300 especially when they invite every other OEM to do it as well.
Yes...
I see your mistake, though. You're assuming that developing a game is just using openGL or DX as an API. No... it's not like that... at all.
They do use these standardized APIs, but they also have room(s) where they've got a multitude of hardware as a test platform. Why would they need this? Well, just because you're using an API that stretches across hardware/platforms doesn't necessarily mean your game works on a completely different set of hardware (and respective drivers). This happens all the time in driver releases. nVidia or Intel will release a new update that addresses a specific glitch in a specific class of hardware for a specific game. The developers have to account for that stuff before it ships. With PC gaming it's a Sisyphean task given the plethora of hardware and exponentially higher hardware configurations. A 'standard API' doesn't account for any of these things.
Furthermore, AMD or nVidia release a GPU that has a set amount of theoretical GFLOPs; I say theoretical (and this bit's important), because that's the theoretical max throughput of that specific GPU. It's impossible to reach the max but with enough attention and time (and lots of work) you can squeeze more and more out of that GPU. So when you're buying your GTX680 and looking at those amazing frame rates, most people think "Wow, is this GPU fast!" when a few of us are asking "Christ, is this all they managed to get out of it?" A 7970 has 4.3 billion transistors! There should be no reason why any game you're playing has less than 60 FPS. The reason some games perform better than others is the programming. That's why 'The way it's meant to be played' games perform better on nVidia hardware (especially hardware and drivers that was given to the developers) while 'Gaming Evolved' games play better on AMD hardware.
This isn't only an issue in PC gaming but also elsewhere as well. Android is a perfect example. If you go to a serious Android developer, you'll see they've got a multitude of devices and hardware configurations. Despite Java being a "write once, run anywhere" language, that doesn't mean your software will behave nicely on X or Y hardware. You need to test, tweak and optimize. This is also one of the reasons why Apple has "faster" applications and even a larger selection of apps.
So, no, it isn't just code and slap a price tag on it and ship. If you do that you end up with games like Rage.