Proposed resolution:

Change the sentence in question to "... the only exceptions are destroying an iterator that holds a singular value, or the assignment of a non-singular value to an iterator that holds a singular value."

Clause [iterator.concepts], paragraph 5, says that the only expression that is defined for a singular iterator is "an assignment of a non-singular value to an iterator that holds a singular value". This means that destroying a singular iterator (e.g. letting an automatic variable go out of scope) is technically undefined behavior. This seems overly strict, and probably unintentional.