The idea that in the first two images the circle stood for Steam OS and that in the third image it's suddenly supposed mean something totally different (controller, Source 2 etc) isn't consistent with their own visual language. A language that so far has been pretty consistent. O=Steam OS. [O ] = Steam OS in a box. O+O= Steam OS + Steam OS.
The new part of the language is the '+' symbol. What does that mean in context of two steam OSs? Are they being combined? What is the result of two combined Steam OSs? Are they meant to be independent but connected? How are they connecting together and for what purpose?
Wasn't someone quoted as saying that SteamOS was not just an operating system, but also a "cooperating system"? If that's the case, it may be that O+O isn't 2 SteamOS's, but SteamOS and some other OS. This is just my wild ass guess, but I'd think that would imply that the announcement could have something to do with some system for loading both SteamOS and Windows/OSX onto the same machine. Or it might have something to do with streaming stuff from your high(er) end PC running Windows to your Steam Machine running SteamOS.
Or it's something completely different, like "every Steam Machine comes with a complimentary Bowie knife", I don't know.