The article doesn't talk about how "hard" a text is - nor does it equate in some simple way the amount of brain activity with how "good" a text is. One of the joys of literature is just that it is, or can be if that's what we are into, about much more than just absorbing stuff, but about being affected by it, it can provide us with insights about ourselves and the world we live in and it can move us with the beauty both of the content of what is expressed and the means by which it is expressed. How it does that is, probably, by activating regions of the brain that less "difficult" texts do not.
That seems to be what the article is saying in the bits of my brain that got activated by reading it.
Men never commit evil so fully and joyfully as when they do it for religious convictions.