There is some truth to this claim although I would not rate the study which I source here as high-quality evidence.
- Chen AL, Chen TJ, Braverman ER, Acuri V, Kemer M, Varshavskiy M, Braverman D, Downs WB, Blum SH, Cassel K, Blum K., Hypothesizing that marijuana smokers are at a significantly lower risk of carcinogenicity relative to tobacco-non-marijuana smokers: evidenced based on statistical reevaluation of current literature., J Psychoactive Drugs. 2008 Sep;40(3):263-72.
Male and female [marijuana and tobacco smokers] had a lower probability of developing lung cancer than did [tobacco-only smokers].
It is a primary study using statistical modeling. It showed correlation, not causation.
Keep in mind that they only looked at lung cancer while smoking tobacco increases the risk for several other types of cancer.
For biological plausibility of these claim I might refer to:
Cannabis smoke contains cannabinoids whereas tobacco smoke contains nicotine. Available scientific data, that examines the carcinogenic properties of inhaling smoke and its biological consequences, suggests reasons why tobacco smoke, but not cannabis smoke, may result in lung cancer.
Although only a side-note, tobacco companies use low-quality fertilizer which may contain radioactive elements:
Edit: I would like to add a quotation by a large meta analysis by Huang et al. 2015. After listing several reasons why smoking cannabis should cause cancer they state that:
In view of the above findings, a null association between marijuana use and lung cancer is somewhat surprising because marijuana smoke contains known carcinogens in amounts com-parable with those found in tobacco smoke (49). Although the generally smaller amounts of marijuana that are regularly smoked compared with tobacco might appear to explain the null association of marijuana with lung cancer, the absence of a dose–response relationship between marijuana use and lung cancer,in contrast to the strong dose–response relationship noted for tobacco (16), would argue against this explanation. A more likely explanation is a tumor-suppressant effect of THC and other cannabinoids evident in both cell culture systems and animal models of a variety of cancers, as reviewed by Bifulco and colleagues (57). These antitumoral effects (antimitogenic, proa-poptotic, and antiangiogenetic) could possibly counteract the tumor-initiating or tumor-promoting effects of the carcinogens within the smoke of cannabis.