It isn't that Sam doesn't know the demons that had been stirred up by James's mistake. But he doesn't expect Castiel to start talking about them. It's not something that he's ever done, something they've ever really talked about.

Sam turns his head, kissing Castiel's temple and holding him close. "I forgave you for that long ago," he murmurs, stroking his hands over Castiel's back. "Well before we thought you were dead, even." Certainly Castiel had to know that, had to remember Sam praying, saying that Castiel was still one of them. "I didn't know you were still conscious, though. I guess...I sort of guessed you might have been or might remember some of it. That's why choking was always off the list even before you said no. I wouldn't have wanted to bring those memories back up."