You'd have to quote directly to prove this.
Because a papal resignation creates a PARTIAL, but incomplete vacancy. The Roman Pontiff is the main part of the Apostolic See, and, so, when he resigns, it creates a partial vacancy. But, as the rest of the quote, says, that type of partial vacancy (one resulting from a resignation) does not annul the need to follow all of the procedures in the papal election law. In that case, the curial officials still act in the place of the Roman Pontiff (in a limited way) until the Pontiff dies. At that point, upon the retired Pontiff's death, the Curial officials lose their offices (except for Camerlengo) and an election is held to elect a new Roman Pontiff.
Okay. I'll try to be more clear. It does take some unravelling.
1.
Universi Dominici Gregis is subtitled "On the Vacancy of the Apostolic See and the Election of the Roman Pontiff." So understanding precisely what constitutes "a vacancy in the Apostolic See" is critical. This is because the "election of the Roman Pontiff" cannot happen BEFORE there is a "lawful vacancy in the Apostolic See," that is, a "vacancy" as it is defined by the Law.
2. By definition, an ecclesiastical office can either be occupied or vacant. Since the Apostolic See is composed of multiple offices (per Canon 361) it can be partially vacant or completely vacant:
Canon 361 In this Code, the term Apostolic See or Holy See refers not only to the Roman Pontiff but also to the Secretariat of State, the Council for the Public Affairs of the Church, and other institutes of the Roman Curia, unless it is otherwise apparent from the nature of the matter or the context of the words.So with respect to the Apostolic See, a complete "vacancy" occurs only when all the offices that make up the Apostolic See are no longer legally occupied by "officials." If any of the offices of the Apostolic See remain occupied, it is by definition not vacant, in the strict sense of the word.
3. How does the Apostolic See become vacant? Does it become vacant on the "resignation" of the Roman Pontiff. No, as
UDG 14 and
Pastor Bonus 6 state, the Curial offices of the Apostolic See only become vacant upon the death of the Roman Pontiff:
UDG 14. According to the provisions of Article 6 of the Apostolic Constitution Pastor Bonus, at the death of the Pope all the heads of the Dicasteries of the Roman Curia — the Cardinal Secretary of State and the Cardinal Prefects, the Archbishop Presidents, together with the members of those Dicasteries — cease to exercise their office.PB Art. 6 — On the death of the Supreme Pontiff, all moderators and members of the dicasteries cease from their office. The camerlengo of the Roman Church and the major penitentiary are excepted, who expedite ordinary business and refer to the College of Cardinals those things which would have been referred to the Supreme Pontiff.Notice that there is no mention that upon a "resignation" the Curial offices "cease from their office." No, only upon "the death of the Pope" do those offices become vacant.
4. So, when the Roman Pontiff "resigns" a kind of partial vacancy of the Apostolic See occurs. And the Cardinals must respect the "rights of the Apostolic See" even after the "valid resignation of the Pope," as we see in UDG 3:
UDG 3. I further establish that the College of Cardinals may make no dispositions whatsoever concerning the rights of the Apostolic See and of the Roman Church, much less allow any of these rights to lapse, either directly or indirectly, even though it be to resolve disputes or to prosecute actions perpetrated against these same rights after the death or valid resignation of the Pope. All the Cardinals are obliged to defend these rights.The above states that the College of Cardinals are legally prevented from usurping the "rights of the Apostolic See" after "the valid resignation of the Pope." Think about that. This means that it is possible for there to be a situation in which the officials that make up the "Apostolic See" have "rights" and at the same time the Pope will have validly resigned.
5. When the resigned Pontiff does finally die, the Apostolic See becomes legally and fully vacant because the Curial offices that make up the remainder of the Apostolic See are vacated (per UDG 14 and PB 6). Then the Camerlengo takes over to prepare for the election. This authority is given again in UDG 14 and PB 6:
UDG 14. According to the provisions of Article 6 of the Apostolic Constitution Pastor Bonus,13 at the death of the Pope all the heads of the Dicasteries of the Roman Curia — the Cardinal Secretary of State and the Cardinal Prefects, the Archbishop Presidents, together with the members of those Dicasteries — cease to exercise their office. An exception is made for the Camerlengo of Holy Roman Church and the Major Penitentiary, who continue to exercise their ordinary functions, submitting to the College of Cardinals matters that would have had to be referred to the Supreme Pontiff.PB Art. 6 — On the death of the Supreme Pontiff, all moderators and members of the dicasteries cease from their office. The camerlengo of the Roman Church and the major penitentiary are excepted, who expedite ordinary business and refer to the College of Cardinals those things which would have been referred to the Supreme Pontiff.6. Chapter 3 of UDG deals with all the details of who is in charge of various things during a strict (complete) Vacancy in the Apostolic See. That chapter is too long to copy here. Just pull up UDG if you want to read all of it. But I will quote UDG 23 to contrast with UDG 3 (quoted above in my section 4) to illustrate the difference in the governing ability of Apostolic See when a Pope resigns versus when he dies:
UDG 23. During the vacancy of the Apostolic See, all the civil power of the Supreme Pontiff concerning the government of Vatican City State belongs to the College of Cardinals, which however will be unable to issue decrees except in cases of urgent necessity and solely for the time in which the Holy See is vacant. Such decrees will be valid for the future only if the new Pope confirms them.Here you can see that after the Pope dies and a full vacancy of the Apostolic See occurs, the College of Cardinals no longer needs to respect "the rights of the Apostolic See." This is because the Apostolic See is truly vacant and only the College of Cardinals are available to govern the Vatican City State. But the College of Cardinals can only govern in a very limited way during the papal interregnum. During the resignation of the Pope (but before his death), the Curial offices of the Apostolic See continue to govern the Vatican City State with full control. After the death of a Pope, the College of Cardinals (not the Curial officials) govern the Vatican City State in a limited capacity until a new Apostolic See is occupied by a new Pope and his Curia.