I think you're probably right, and I didn't want to make my point about general intelligence, but only focused on food knowledge.
It's hard to prove any food healthy or unhealthy, but one of the main premises of healthy eating is prefering fresh ingredients over highly processed food. The studies which support that view are large-scale and wouldn't be able to point to a specific problem with Soylent, but undoubtedly Soylent is very much on the processed side in the spectrum of foods.