You are right, one should consider preparation time per serving. In your example something like 2 to 3' per serving which is more than reasonable.

When our life became busier (young kids + work), we started to cook 2 hours or so every week end, several recipes at a time. We order vegetables from a local producer (no fancy app, just a excel sheet) on thursday, get them early on saturday, and then cook stews, soups, meals that will feed us for the rest of the week. This requires some planification and organization but makes the whole cooking time argument irrelevant.

The other thing that bothers me with soylent enthusiasts is that they seem to disregard how the ingredients are sourced and their environnemental costs. The website's FAQ "From what is it made" links to the nutrition facts.