It would to be an extraordinary ending for a tragedy, two men who'd been allies and enemies both locked in the same coffin, continuing the same struggle. It is a tragedy, that Lan Xichen is still living through -- only the story of it has a conclusion, because stories always do. Lives keep going.

But (Claudius thinks) a mystery always ends with some sort of parlor scene, where the detective explains everything that's happened. That's why Claudius asks, "So how did Wei Wuxian discover Nie Huaisang was behind it?" He has a fair guess it was Wei Wuxian.