a central focus in the film is the role of poor nutrition on the lifestyles of the future, and the dominant effect of fast food on our diets.

it is both the case that the flynn effect (which is what he's talking about) is usually explained primarily via increases in nutrition in our diets (as a result of oil) and that poor diets, especially diets high in fast food, lead to lower iq scores.

so, if he's projecting a future where people have been eating garbage for centuries, we would expect the flynn effect to have undone and reversed itself quite some time before the film takes place.

https://healthyeating.sfgate.com/relationship-between-fast-food-brain-12203.html