Damage to the prefrontal cortex resulting in cognitive inflexibility can result in a myriad of fixed beliefs—they’re not necessarily religious in nature.

And religious fundamentalism is a particular type of extreme religious belief; most people don’t hold to fundamentalism but are nonetheless religious, so the study doesn’t account for anywhere near all religiosity and certainly doesn’t refute the point that religious faith isn’t a form of mental illness.

I want to make something clear here: I’m an atheist and an antitheist, but I’m also a therapist and it really irks me when atheists try to conflate mental disorders with religion. It’s an example of atheists fueling their distaste for religion by giving in to amateurish ignorance about psychology. Learn what the fuck you’re talking about before trying to make claims that go against what all of the experts in a field of study agree upon. Honestly, atheists ought to know better.