In this case, the parent comment used "richness" as a synonym for being diverse and high quality.
on edit: just seems a weird word to use when that has a precise meaning for food, if they had said a richer selection of food I would understand. But as the discussion was about what makes Americans unhealthy with shorter lifespans, and the unhealthiness of Americans is often synonymous with being overweight, saying it's because Americans eat rich food just doesn't seem to make any sense if what they mean is Americans have a wide selection of food to choose from.