While nullptr is a null pointer constant, it is not a null pointer value. The latter is a value of some pointer type, which std::nullptr_t is not.
Reference:
A null pointer constant is an integer literal (5.13.2) with value zero or a prvalue of type
std::nullptr_t. A null pointer constant can be converted to a pointer type; the result is the null pointer value of that type and is
distinguishable from every other value of object pointer or function pointer type. Such a conversion is called
a null pointer conversion. […]
7.11/1 in N4659, emphasize mine
So NULL can indeed be nullptr without providing the arithmetic operators.