Why? Their kiss has nothing to do with Lu Tien and Uncle Iroh.

Leaves from the vine is a direct reference to Iron's character and/or his relationship with his son. the avatar's love was LITERALLY created and named for aang and katara. "avatar's love"

Plus the meaning of leaves from the vine is about a son going off to and coming back from war (or dying since "leaves from the vine falling so slow" seems to be a allegory to young soldiers dying")

I have a very strong opinion on this. Like @AvatarPhoenixGrey2 and @Bastion2.0 said, the kiss had nothing to do with Uncle Iroh and Lu Ten. I think it would be very upsetting if that song was played.