It's profoundly silly to talk about an Open Amiga without actually opening up AmigaOS! What's open, the case cover?
Every single project hosted on the Open Amiga site is open-source, and will remain that way (as per their open-source licenses); that's what's open. It is hoped that Hyperion will like (some of) these projects, and choose to make them an official part of the OS. If Hyperion do so then these modules will still remain open-source, even if other parts are still closed. This is rather similar to Mac OS X having large amounts of open-source code, while other parts are still closed.
I honestly don't get all the fuss. This site was created years ago in response to members of the community wanting to develop open-source OS modules that might eventually become official OS components. Recently, there have been people voicing their interest in helping to improve OS4, and the Open Amiga website is one possible avenue. Steven's blog post just highlights this site as an option, and I guess hints rather strongly that Hyperion has been paying attention to this community effort.
Hans