What they found is that lung cancer cells left in a test tube containing green tea became "stickier" and therefore less like to metastasize. (Its proclivity to metastasize—or to spread throughout the body from the original cluster of cells—is what makes lung cancer so deadly.)
The mechanism by which green tea makes the cancer cells stick together most likely has to do with proteins: Some compound—or combination of compounds—in the green tea changed a critical protein on the cancer cells' surface, making them stick together.
Although this study does not prove that green tea prevents lung cancer, it does indicate that further research is warranted.
This report also brings up an important point about green tea extracts. Extracts are usually made to isolate specific compounds, such as the well-known antioxidant EGCG. However, the extract-making process may leave out compounds that are unknown today but which may someday be shown to have amazing health benefits.
The moral of this story, grasshopper, is Never second-guess Mother Nature.
Besides, by drinking brewed green tea, you're also hydrating your body. And brewed green tea tastes better than any pill!
So drink freshly brewed green tea—because you never know what you might be missing.