I think what Sonnen thought was AS breaking was actually him relaxing. During the fight you can see AS take some deep breaths and close his eyes. I can very easily see how Sonnen on top of him would feel like this man is broken. Then again shortly after Sonnen passed guard and even got to mount, so all facts during the fight would have concluded that Sonnen was right from his point of view.
I can easily see why Sonnen thought he broke him. It doesn't make it the reality of what happened, but I can see his point of view.