It's fairly obvious you have to have the announcement first, then the rollout on major sites (that aren't run by Google.) If the reverse happened, it would be seen as a foul-up. Facebook did the same when they announced "Likes" on external web sites.
In fact, I've checked all the sites listed in the article - none of them have Google+ sign-in.
Also, they say you sign-in via Google account, but I suspect it also requires a Google+ profile in order to use this feature.