You can't make claims like this without context. There's 85th-percentile health, and then there's 99th-percentile health. Plenty of foods, including Soylent, can help people attain or maintain that more modest level of health. Now, if you were an Olympic athlete requiring 99th-percentile health, I agree you'd be insane to make Soylent more than 30% of your daily calories. But if you're an average person eating the Standard American Diet, you are currently insane from a health standpoint, and switching to Soylent would make you a lot less so. And among all the options you might have for improving your diet, it is among the cheapest and lowest-effort.
It amazes me how people can fail to recognize that nutrition is a part of a larger economic problem of time and resource allocation. There are real tradeoffs to be made! Perfection is rarely attainable or even desirable except in special cases!
Rob Rhinehart, for his part, has always been honest about this. The Soylent team has looked at including things like antioxidants and digestive enzymes--things you'd get from whole foods but not from Soylent's processed ingredients--but they concluded that there's just not enough information to determine the types and quantities of these substances that are necessary to improve health. I think this lack of information is no accident. It's easier to research core nutrients whose deficiency will cause clear symptoms than it is to research nice-to-haves like antioxidants whose presence may cause slight performance improvements at the margin. Soylent is designed to reflect the state of nutritional knowledge. As that knowledge improves, so will Soylent. While I have bought MealSquares and enjoy their product, I prefer Soylent's approach to one that pretends our knowledge is more complete than it really is.