The mice need to stand on fork to use the grape as a weapon. The same section says in one sentence, that rat can throw grape, if he starts his turn on space with grape. BUT few words later rulebook says, that it use grape using the same set of rules as mice.
Now, I assume that intention of author was, that rats don't have to use fork to throw grape. But still, there is some unclear contradition in rulebook about it. Watcha think?
I think it's pretty clear. When the rats use the grape as a weapon, use the same rules as when the mice use the grape as a weapon. It's just that as a precondition for being able to use the grape as a weapon the mice must have the dinner fork. The rats don't have that restriction since, obviously, they cannot ever have the dinner fork.