The list of known carcinogens is quite short. That is mostly because it is difficult to conduct studies with a large enough sample to be sure that something is a carcinogen with high statistical reliability.

Benzene is certainly one of them, and it's present in cannabis smoke (well, all smoke from burned plant matter, so cannabis included).

Otherwise I'm not sure what your point is. They wouldn't need to identify specific compounds to establish a strong correlative relationship between lung cancer and cannabis. Such a relationship has never been established, and not for lack of trying.