Several studies have shown an inverse correlation between intake of garlic and risk of cancer in the digestive system (see this review for instance).
I could not find other studies looking investigating the intake of RAW garlic and its effect on lung cancer. In the chinese study you discuss, they suggest that the vapours released from eating raw garlic are part of the preventive effect.
Several mechanisms have been proposed to explain the chemopreventive effects of garlic and related OSCs, including inhibition of mutagenesis by inhibiting the metabolism, blockage of DNA adduct formation, free-radical scavenging, effects on cell proliferation and tumor growth, and modulation of immune responses (42). The volatile oil with effective OSCs are largely excreted via lung that might be related to above reviewed mechanisms or functions, in addition to its role in infection control, inflammation reduction, and probable protection of lung from carcinogens such as polycyclic aromatic hydrocarbons. These experimental evidences are supportive for the hypothesis of the protective effects of raw garlic consumption against lung cancer.
The study in general seems to hold up, but I am not an expert in this particular field so I may be missing something vital.
Note that this is however a CHINESE study. As the reason for the beneficial effects of garlic are not fully understood, there is NO evidence that a person living somewhere else would have a lower risk of lung cancer. A population based study is only valid for statistically similar populations. For instance they mention that 49.6% of the lung cancer patients and 50.8% of their control group are illiterate. In comparison the literacy rate in the US is 99%.
Another thing that might be of concern is genetics. It is known that Asians with lung cancer have a different response to certain forms of therapy than other population groups. I was unable to find a study that investigates if similar effects are seen with RAW garlic.
Also note that the correlation they find is for SMOKERS ONLY. They simply don't have enough non-smoking cancer cases to say anything about non-smokers (p-value 0.137):
In stratified analyses, a borderline inverse association of raw garlic consumption with lung cancer risk was observed among never smokers, which is most likely explained by small sample size of nonsmoking cases.
The same goes for exercising. They can't find an effect in people who exercised regularly 10 years prior to the study (p-value of 0.332).
As eating raw garlic is fairly uncommon in western countries (could not find a source for this, based on personal experience), it would be hard to confirm the study. A US study would likely need thousands of participants, all eating raw garlic for a decade.
A more likely scenario is the isolation of the active ingredient in the garlic, which could then be used as a supplement.
TLDR: If you are living in China, smoking and not exercising, eating raw garlic more than twice a week is likely to reduce your risk of lung cancer.