What isn't clear to me from that post is whether they're giving something for the open source community, or for a part of the Linux community that runs proprietary drivers, distribution platforms, games, etcetra on top of some open source?

It does not state on what drivers/other stuff they are contributing. It sounds they work together with the vendors to improve the proprietary drivers, I think. That would be zero benefit for GNU/Linux environment. Also current state of Steam OS is just a standard Debian with proprietary components added. And those components are what really matters in this field. I do not expect that Valve make their Steam Client open source, but at least not relying solely on closed stuff for the relevant parts. In the current state this doesn't improve anything or makes somehow the "ecosystem" better.