Lilmod,
That is a fine distinction, and you might even be correct that Rav Moshe would hold that smoking would not be assur for that reason. But Rav Moshe is not clear on whether length of time before the activity causes death, by itself, could make something mutar. Therefore some have argued that Rav Moshe basis for being matir is primarily based on the risk of smoking being the ultimate cause of death being less than 50%.