Actually, the bowels themselves don’t really see any difference, in terms of what they’re moving. By the time food gets there, it’s already liquid, Soylent or not. But as far as breaking down less “readily available” nutrients, there’s probably a valid point in keeping some “regular” food in your diet.
On the other hand, this could also be at least partially psychological. A lot of people are comparing post-soylent “real food” to pre-soylent “junk food”. When you adopt a new routine, your mind adjusts to see that as the normal – so to some degree, it could just be that compared to soylent, the body is noticing the (comparatively) lower quality of other foods. Not saying it’s entirely in your head, because I’m sure it’s not – just that it’s almost certainly a factor.
Or to draw an analogy, if you lose 30lbs over time, you don’t feel that much lighter, but if you put back on a 30lb weight vest you’d sure notice it (and, if you kept the vest on for a week, you’d probably stop noticing it)