It may be undefined behavior to explicitly dereference nullptr, but the point was to show that a reference derived from a pointer could still be invalid, and it's a real situation that a programmer might encounter.

A more important point is that the standard uses very strong language to guarantee that a reference (`&`) is valid.

Forming a reference to an invalid object, by playing with a pointer or otherwise, and using a reference after the associated object becomes invalid exhibits behavior which is not defined by the standard.

The possibility is only real for broken code, and you can't generally predict how your code may behave if handed such a manufactured reference in any event.