Why would they put he died of smoking on the death certificate if he actually died of an embolism secondary to lung cancer. He stopped smoking 15 years ago. This doesn't make any sense.

embolism secondary to lung cancer
Does NOT mean he died of smoking.
A smoking related cause, but nit actual smoking.

I can't conceive of any doctor specifically listing smoking as the cause of death. But the lung cancer may have been a result of smoking.