"Dyw hi ddim yn hoffi e o gwbl" is also expected as a correct answer (in the check-all-correct-answers exercise). Where is this form mentioned or explained in the grammar notes? I can't find it anywhere.
"Dydy" is north waliau'n whereas "Dyw" is more typical of the south. They come from the more traditional form "Nad yw" and "Nad ydy". EDIT: Shwmae is correct. Bad habit of mine using "Nad" instead of "Nid".