I believe the point is that a sandwich doesn't have the macronutrient ratio most people want and a full range of micronutrients so people don't have to be worried about missing anything important if they decide to eat sandwiches for 2/3rds of their meals.

Isn't this why we invented the "multivitamin"?

Research is increasingly pointing out that vitamin supplements don't work for certain nutrients.

Isn't Soylent equivalent to a supplement in essence? It's a base diffused with certain nutrients. Why is this different than a sandwich and a vitamin in terms of nutrition? Or even more, how is it different from a vitamin?

In a word? No.

+1. I could eat a sandwich every day for every meal. I don't, but I certainly could.

A focus on fat in the morning, then vinegar in the afternoon and finally umami in the evening would serve me well. All between 2 pieces of appropriate bread.

Yes