Interesting find!
I think a lot of us would agree that smoking is not exactly a DISEASE but rather a lifestyle choice. However some would argue (as in alcoholism or drug addiction) that tobacco addiction is not different than a disease. As a non-smoker, my preference is to characterize it as a lifestyle choice.
That being said, tobacco use disorder has an ICD-9 code: 305.1, and I'm certain that would be their argument is that it's a defined disease entity even though it's not commonly thought of as a disease or "pre-existing condition."
Really though, what will they think of next to deny claims or even deny coverage? The insurance industry has hit yet another moral low.