What is the cause of a protuberant belly in kwashiorkor?
The Gastroenterology paper you cite doesn't mention distended bellies at all.  There is no dispute that fatty liver is a symptom of kwashiorkor.  But the Wikipedia article makes it quite clear that the cause of the distended belly is edema.


  The extreme lack of protein causes an osmotic imbalance in the gastro-intestinal system causing swelling of the gut diagnosed as an edema or retention of water.[4]


And:


  Compromised fluid recovery results in the characteristic belly distension observed in highly malnourished children.[14]


Here's another source:


  To understand the reasoning for this, it is important to know that in malnourishment, the rounded abdomen is not due to fat accumulation. Instead, the water retention and fluid buildup in the body cause the abdomen to expand. This results in a bloated, distended stomach or abdominal area.


The rest of the article goes into more detail on the phenomenon and mechanism of action.

This source seems less authoritative, but also suggests that weakened abdominal muscles (due to protein deficiency) also play a role:


  The swollen belly is the result of not only that enlarged liver, but also the loss of muscle mass, which leads to weak abdominal muscles. Together with a weakened lymphatic system that is unable to keep up with the body’s liquid wastes, when those wastes accumulate in the abdomen, the belly becomes distended.


So the author suggests that fatty liver contributes to belly distension, but is not a sole cause.  None of the sources I looked at mentioned fat accumulation as a contributor, but rather eliminated it.