Fiber can help form stool if you have chronic diarrhea, but it can also help relieve constipation.
You are correct that diuretic refers to peeing; what the poster was probably trying to say was "laxative." And foods high in fiber and sugar can certainly work as laxatives!
You are correct that diuretic refers to peeing; what the poster was probably trying to say was "laxative." And foods high in fiber and sugar can certainly work as laxatives!