The study referenced in the OP is similar to many other very good studies that suggest, for large populations, there is a correlation to a strong negative correlation between IQ and religiosity (the higher one's IQ the less likely they are to be religious). This isn't anything new. Having said that, academic studies rarely draw conclusions that show direct correlations/cause and effect.
These studies show correlations. Of course there are intelligent and brilliant religious people and of course there are brain dead atheists (probably not but I'm saying that just to make a point). It should not come as a surprise. Let's put aside the evidence that IQ and religiosity show a negative correlation for a moment.
Let's focus on the very religious. Anyone who allows someone else to do their thinking and has given up all analytical rigor, swallowing the tepid confirmationally biased nonsense spewed by their leaders and relying on their feelings and beliefs for many years as do so many very religious people, is not using a vital muscle - their brain. It isn't any different than a couch potato. Unused muscles atrophy.
Look at the Dubs over the past few decades. Deep study and analysis, which was encouraged for a while, rose up and bit the Dark Lords square on their fat asses. To mitigate this they have dumbed down the cult, discouraged deep study and began appealing to emotions and feelings in order to keep the rank and file in line.
This isn't rocket science.