I don't see how this could be true. It would be analogous to observing a species of bone-thin weaklings that becomes interested in body building over the course of a few hundred years, gaining more muscle mass on average with each passing year, and making the claim that the strength of this species has not changed. Maybe if one of the early weaklings decided to take up their own interest in body building, they may have reached a similar strength to that of their descendants (though even that is debatable since that specific individual wouldn't have access to all the training techniques and diets developed over the course of its species' future); however, it seems like an awkward interpretation to say therefore the strength of the species has not changed.
This is similar to the situation we find ourselves regarding intelligence in the human species. Humans gain intelligence by exercising their brains and engaging in mental activity, and humans today are far more occupied by these activities than our ancestors were. This, in my view, makes it accurate to claim that human intelligence has changed significantly since the advent of religion. Individual capacity for intelligence may not have changed much, but the intelligence of humans as a whole has changed.
Note that my argument does not conclude that human knowledge or understanding has changed over time. These attributes certainly have changed - I'm sure not many would doubt that. It also doesn't conclude that every modern human is more intelligent than every ancient human. Instead, it concludes that human intelligence as a whole has changed as a result of changes in our culture that influence us to spend more time training our intelligence than our ancestors.