>One of the nootropics measured, choline bitartrate, has carefully regulated ability to cross the blood-brain barrier beyond a certain level, and so it would be surprising if it had direct and immediate cognitive effects.

This isn't quite right. This is true of choline bitartrate itself, but IIRC it is fairly quickly metabolized into other forms of choline that can cross the BBB.

I would say it's pretty likely that increasing choline intake would have a significant effect on cognition. Humans have been consuming large amounts of fish for much of our evolutionary history, compared to today's typical diet. Given the way choline is used by our brains, and the relative plethora we had available when our brains were evolving, it's hard to believe that we get as much as we can use in a modern diet.