I doubt that that study was able to tell whether the dietary changes improved nutrition. They don't appear to have looked at many nutrients, or figured out which nutrients the subjects were most deficient in. Even if they had quantified all important nutrients in the diet, nutrients in seeds are less bioavailable than nutrients in animal products (and that varies depending on how the seeds are prepared).

There's lots of somewhat relevant research, but it's hard to tell which of it is important, and maybe hard for the poor to figure out whether they ought to trust the information that comes from foreigners who claim to be trying to help.

I'll guess that that more sweet potatoes ought to be high on any list of cheap improvements, and also suggest that small increases in fruit and seafood are usually valuable. But there will be lots of local variation in what's best.

I think peanuts would be a significant part of it, as peanuts are healthy and cheap to produce:

https://www.stack.com/a/are-peanuts-healthy/

Didn't they also write in Poor Economics that a basic meal would include many bananas and eggs? Ideally, you would tailor the food to what is available and cheap in the region. This constrained optimization problem (minimize costs while maintaining a certain amounts of nutrition groups) is something many nutritionists do, especially those that are in charge of providing balanced meals for the elderly. I can imagine doing the same for the prices of food.