Co-occurence is not the same as causality. To assume so is to demonstrate, at least, that you are ignorant of the fundamental nature of statistics.
Intelligence tests are not categorically descriptive of intelligence. There are things that they poorly measure. They cant measure genius at music, or horse-back riding. They only measure performance within a narrow window of cognitive function.
Your study, because its a poll, could have a self-selecting demographic such that intelligent religious people were disinclined or even given a negative incentive for participation.
Consider that many of the greatest thinkers within "modern thought" were religious. Newton was moderate but religious and his calculus, mechanics, and optics are the foundation of physics. Anyone who asserts that he was anything less than a genius isnt in a hard science.
Consider Srinivasa Ramunajan a mathematical genius of this century. He did things in the upper stratosphere of mathematics that are categorically qualifiable as genius quality. He was a devout hindu, and travelled to england only after having a vision that his families "god" came to him and said he should.
These two counter-examples at least disprove any categorical claim that religiousity and stupidity are causitively coupled.
Those are "cake" counter-examples. There are hundreds more. They beg the question: why does it matter to you? Why is it important to assume there is a causal connection between "religiousity" and "idocy"?
The answer cant be a rigorous pursuit of truth, can it? It cant. Maybe you are religious in your own way, and like all religions you are thumping your chest and saying "we are the only true church, all others are false". Its not a new message.