This rumour just won't go away. But this particular report is even less credible than the previous one.
Firstly the value of $1bn, ok now one billion dollars sounds like a lot but it really isn't. Google themselves bought Youtube in 2006 for $1.6bn when Youtube was basically nothing. At the time it was a small site, that not really all that many people used.
Twitch in 2014 is a totally different prospect. It's the 4th most visited site in the US, and is growing rapidly, pulling in tons of ad revenue and other money, so there's no way it's worth as little as a billion. It's real worth, if it was up for sale would be many times that.
The other glaring problem with this story, if you go to the source linked by the Yahoo report is that in that story they prominently mention that Google and Twitch are appearing at an upcoming event, which by a staggering coincidence is run by the site carrying the news report. If it's just an attempt to generate interest in their event, it's working!
Having said all that, I wouldn't be surprised if Google have considered taking over Twitch, because at least on their side, it makes sense. But until something is announced officially by Google or Twitch, this is still only a rumour.
TLDR: Dubious source, probably still false rumours, wait for confirmation from Google and Twitch