A low-fat diet is good for rabbits, but they digest fiber differently than humans do. Fiber does not cause bloat in rabbits, it's actually a good preventative for it. Humans get bloat from fiber because our seldom-used fiber eating microbes go into overtime and produce lots of gas. This decreases in time if you continue with a high fiber diet.
Rabbits on the other hand are designed to eat foods that are very high in fiber and low in nutrients. A diet too low in fiber can actually cause GI stasis or make it happen more frequently. Gas buildup in rabbits is more often caused by a GI slowdown which prevents the gas from moving through as normal or bacterial overgrowth from to many carbohydrates or fast changes in diet.