The example was simply to show that you don't need all pointers to be nullable to have linked lists. The advantages for this case are minimal, since linked lists specifically depend on the 'next' pointer being nullable. However, linked lists are a hardly representative of pointers in general, particularly in a language like Java where all objects are accessed via pointers. The benefit of making pointers non-nullable by default is in the majority of cases where nulls are nothing but a source of runtime exceptions.
The advantages tend to be more pronounced in languages like Haskell, however, which have more advanced type systems which handle this sort of subtyping without any need for runtime casts. As you point out, in Java you would need to explicitly cast from List<T> to ListNil<T> or ListCons<T>, an operation which can fail in much the same manner as dereferencing a null pointer.