Vague statistical inference can not possibly establish such a causal link. Even valid associative inference should establish a 50-100% correlation between smoking and cancer, but it does not even come close. Most people who smoke don’t get lung cancer, and at least 10% of Americans who do get lung cancer- do not smoke. There are also huge international/ethnic variations among smokers and cancer rates. There is currently no proof whatsoever for the alleged smoking-cancer causal link. None. Smoking is a disgusting and silly habit. But all that one can now objectively say is that it is a risk factor for cancer and increases the incidence of lung cancer.And this was not the only person in the comments who was casting doubt on this association. As an epidemiologist, I want to scream. If people will not believe this evidence then they really will not believe any level of evidence for observational epidemiology. We have cohort studies going back 50 or more years (Richard Doll has one). Even better, the members of this cohort did not initially know that smoking was harmful (and I recall that the original hypothesis was automobile fumes and not smoking, although my memory may be failing me here). So we don't even suspect a healthy abstainer effect.
The requirement for a 50 to 100 correlation seems to ask for smoking to be directly causal of lung cancer instead of increasing the underlying risk of lung cancer. Consider skiing and broken legs. Not all broken legs are due to skiing and many people ski and do not break a leg. But there is no question that skiing is a risk factor for broken legs. Another good example is collapsed disks in the back. If you are working with a veterans population, the first question you ask when you see a compression fracture in the spinal cord is "were you a paratrooper?". Not all paratroopers have compression fractures and not all compression fractures are due to jumping out of airplanes, but it is a pretty direct link to increased risk.
There is a libertarian line of defense here: people ski because they value the enjoyment of skiing more than the risk of a broken limb. I am not always delighted by it, but it is at least an arguable position. But directly denying the link between smoking and lung cancer seems to be setting a very aggressive standard of proof.