Ok, maybe they don't speculate on that hypothesis for very long. They also looked at nut consumption other then peanuts, and saw a similar protective effect, so believe it was polyphenols (chemicals common in spices, berries, chocolates as well as nuts), fiber, vitamins, and minerals in nuts may confer this protection. They are unclear on the exact mechanism. In this study, it did not seem to effect all types of stomach cancer, and had no effect on esophageal cancer. Other studies had seen decreased risk of colorectal, endometrial, lung, and pancreatic cancers from peanut and nut consumption, this was the first study to look at stomach and esophageal cancer and nut consumption directly.
The conclusion from the article:
"Among older American adults, both nut and peanut butter consumption were inversely associated with the risk of gastric noncardia adenocarcinoma."