The central MacGuffin of the novel is the claim by a Russian informant that the Soviet Union’s entire nuclear arsenal is a vast Potemkin Village: that the USSR’s tech is so shoddy that it is doubtful that more than a tiny percentage of its nuclear warheads would actually make it to their intended targets. Naturally U.K. intelligence wants to confirm this; it seems too good to be true, which drives the rest of the plot. Does the novel ever say whether this is true, or deliberate misinformation, or is the question left hanging? I presume the informant believes it’s true, but the question is whether he was scammed by Soviet intelligence.
I can’t answer the question definitively since it’s been a long time since I read the book, but I’ve read a lot of le Carré, and I would say that the central theme of his novels is not the nuts-and-bolts aspects of the spycraft, but the human toll that it takes on the characters.
You yourself referred to the concept of the USSR’s tech being “so shoddy that it is doubtful that more than a tiny percentage of its nuclear warheads would actually make it to their intended targets” as a “MacGuffin” and I think you’re right. I bet the question was left hanging, just as in the movie 'Pulp Fiction" we never learn what the shiny stuff is in the briefcase.