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Statistics.

Not all smokers get lung cancer. But with large enough samples, smokers turn out to be much more likely to get lung cancer than non-smokers. That's why we say that smoking causes lung cancer. It's not as if someone has actually watched a particle of tobacco interact with someone's lung cells to turn them cancerous in real time.

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Smoking isn't a contagious pathogen.

Second- and third-hand smoke mimic contagion.

That has nothing to do with contagion, that's just a sign that the impact of smoke inhalation isn't limited to the one sucking on a cigarette.

Contagion would mean that the smoke can infect the host, replicate, and spread to others. That's not how second hand smoking works.

Why doesn't that get dismissed with the usual universal dismissal "correlation != causation"?

For one I'd assume its because contagions and physical damage are quite different. That's not to say that correlation between viral presence and disease can't indicate causation, but it is more clear when you are looking at the direct damage to lung tissue from inhaling hot cigarette smoke for years or decades.

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