Benedict made certain to avoid renouncing the Munus (spiritual aspect ). He only resigned from the Ministerium (day to day practical Administration because the people around him were undermining him and disobeying him. So there was no point.

The Apostolic See is impeded, means that the Pope cannot act as Pope for some reason, either external coercion, or there is no pope, or the pope refuses to act out of some irrational or rational conviction. This ‘being impeded’ causes a state of necessity, because the visible head of the visible Church is for all practical purposes non-functioning.