Valve policy is against exclusive. In fact, they want their products to reach the maximum possible platforms. This is essential, because it makes the same investment has much more potential audience.
On the other side, that HL3 is a highly anticipated game ... It's the joke more than anything else. There are those who want to play, yes, because the saga is pretty good, but nobody is anxious to other games as advertised. In addition, fans, if that would make HL3, wait while die for hype as if it were a work either Molyneux.
The reason Valve strives to boost steamos is abusive policies of Windows 8 and later. They have never been very permissive policies of MS, but the issue of application stores and other pose a serious problem for Steam. Not because Valve has not want that because, in fact, has urged other companies to draw their stores steamos.