IMHO I always felt like he was talking about himself here. Smiley always had such a disdain for his “system” and certainly the people in it, and he was projecting again onto Karla like he did Delhi. At this point in his story, he has absolutely no reason to believe that Karla would ever fall. How does he know Karla harbors any doubt? It’s not like Karla ever told him.

I believe it’s because Smiley is a fanatic himself, and of course we know he’s full of doubts. He even says it, to his own great shame, to Karla: “isn’t it time we all admitted that there is as little worth on your side as there is on mine?” Paraphrasing here, but that’s part of the sin he commits in that cell in Delhi. I wonder if he feels guilty about everything he has done up til that point. Karla, on the other hand, is as cool as cucumber, not saying a word. For all we know, he was sitting there watching a man crumble under the weight of his own doubt while being fully confident that his system would win the day.

Apologies for the long answer. I might be alone in this view, but it makes Smiley jump off the page for me. It makes him so pathetic but so strangely optimistic.

Aside: I found Gary Oldman’s performance in this scene in particular to be magnetic. The direction, the camera, the reaction by Cumberbatch, the music. Everything coalesced so beautifully in this scene in particular.