Parent means something closer to pointers (in C++) when they say "reference type". In other words, if a type contains null, then we must always check locally that values of that type are not null before it is safe to use them (or manually maintain contracts).

With option types, we check whether it is empty once, and if it is not, we unwrap it and use it as if it were never nullable. Any function that uses the unwrapped value downstream is oblivious to the fact.