"You had doubts that you were the Wei Ying I waited for, but I do not," he responds. Because he needs to make that very clear, as Wei Ying had felt on rocky standing, but that was before he could see into Lan Wangji's heart and feel the depth, the reach, the sheer overwhelming mass of his love for Wei Ying. He reaches for that worry between his eyes, and presses a finger there as Wei Ying had gestured towards his nose once.

"I love all Wei Ying, but he is not you," he repeats. "I chose you. And I will choose you every time." Which, honestly, probably the most shocking thing about the kiss, but he also doesn't regularly kiss Wei Ying while he's blindfolded and against his permission either. If Wei Ying has asked if he kissed the other one, he can at least truthfully explain that it was because he was a rabbit.

It's also a little rude to say because they don't know if the other one is gone forever - it would break Lan Wangji's heart, but at least, now, he feels liberated to grieve.

In either case, he probably should have told Wei Ying earlier, which is the story of his life, but at least they can have an honest conversation about it, except the part where his soul twin already asked for permission and this misunderstanding is probably what started this entire situation that has Lan Wangji so incensed.