It's mentioned in footnote 6:

As an example, to make this work I’m assuming some kind of “true deref” trait that indicates that Deref yields a reference that remains valid even as the value being deref’d moves from place to place. We need a trait much like this for other reasons too.

It would only work for references that are stable after the value they reference is moved. Think for example of a &str you get from a String.