Atheists & Agnostics

The claim isn't 'all religious people are stupid', it is that there is an inverse correlation between intelligence and religiosity. So yes of course some religious people are smart, some stupid, and most in the middle. But when compared to non-believers, as the article states, the average religious person does worse on various cognitive tests than the average non-religious person.

"As predicted, the atheists performed better overall than the religious participants, even after controlling for demographic factors like age and education. Agnostics tended to place between atheists and believers on all tasks. In fact, strength of religious conviction correlated with poorer cognitive performance. However, while the religious respondents performed worse overall on tasks that required reasoning, there were only very small differences in working memory."