I'm not sure what train of thought you're following here. The Valve announcement is basically that Steam is expanding beyond games, so you can get (for example, although no titles have been announced) Microsoft Office from there.
It was more clutching straws, I was thinking that if Steam were making more games available to Mac consumers it could be something to think about as their has been a lot of talk about better (gaming) GPU's in the next iMac.
But the story doesn't say anything about "making more games available to Mac".
The article is about expanding the Steam market "beyond games". In other words, Steam wants to offer non-game apps. This would have nothing to do with the GPU on an iMac.