Your conclusions aren't based on the article or you miss read that article.

Previous studies have shown that larger groups of primates with more complex social structures are correlated with larger brains. In fact, scientists have used that idea – called the social brain hypothesis – to explain why humans and certain other primates like chimpanzees and bonobos have bigger brains than other primate species. (Now, diet is thought to have played a big role in making human brains bigger than any of our primate cousin's. As we've reported before, scientists think eating cooked meat gave our bodies some extra energy to fuel the building of bigger brains.)

But the authors of the new study compared body size, diet, and social lives (factors like whether they were solitary or lived in pairs, monogamous or polygynous, and the size of their groups) of these various primate species to their average brain sizes. Overall, diet appeared to be a more consistent predictor of brain size for a species than social complexity — brain sized increased with fruit eating more consistently than with greater number of social connections.