He actually spent the first two months on a Soylent-only diet, and only recently added social eating. I think he said something in his three month blog post about a week he spent eating normal food, and he ended up feeling way crappier.
Sure. But 2 months is not long enough. Some unaccounted-for vitamin with a long half-life or a low requirement could give deficiency symptoms after 4 months but not 2.
Also, people on restrictive diets post all the time about how crappy they feel when they reintroduce something. For him to slide comfortably into the explanation "thus my product makes me feel better than restaurant food" is typical of such dieters' enthusiasm.
Although bad restaurant food does exist, much of the digestive upset people experience when going out to eat is simply down to overeating, late eating, or alcohol.
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.