Out of curiosity, if Soylent really does provide every nutrient the body requires, would that mean a user would no longer have to defecate? My reasoning is if every nutrient is absorbed and used, there is no physical waste left, and all toxins and byproducts would be expelled through urine.
Having done pure liquid diets (with significantly reduced caloric loads) on numerous occasions (not by choice, unlike these fellows) this is mostly true. One will typically see bowel movement frequency drop dramatically, although not entirely to zero in my case.
That said, I am missing a significant length of digestive tract, so my experience may be atypical. I would also note that that as calories increase, to or above your metabolic needs, that this effect would likely diminish.