It’s a disciplinary document, so it is not irreformable. Any papal document, including a Bull, that definitively proclaims some truth to be believed or held by the whole Church (which is often done in the form of a solemn condemnation of an error) is considered infallible and irreformable. Otherwise, its not. Furthermore, this is not to give some new doctrine, but to confirm that irreformable truth that has been handed down. The doc in the OP is pretty clearly laying down election law.
The document cited in the OP is actually quite the novelty in how strict it is–barring even those who have repented of their heresy. If this were some dogmatic teaching, Pope Pius II could never have been Pope–he was a concilliarist heretic who participated in a schismatical and heretical council and was the Master of Ceremonies in a schismatical and heretical conclave that elected an anti-pope. And he became Pope.
Anyway many Popes issue legislation governing elections of their successors. It bears pointing out that Pope Pius XII’s legislation (which repeated what most others had done and which as been repeated by his successors) suspends all excommunications and penalties and allows such a person to both vote and be elected. The point of this is so the winner of the office is not contested leading to the kind of confusion sedevacantists try to sow in this regard.
“34. No Cardinal, by pretext or reason of any excommunication, suspension, in-terdict or other ecclesiastical impediment whatsoever can be excluded in any way from the active and passive election of the Supreme Pontiff. Moreover, we suspend such censures for the effect only of this election, even though they shall remain otherwise in force.” (Cons. “Vacantis Apostolicae Sedis,” 8 December 1945)
(passive election refers to being elected, rather than electing someone else).
Subsequent Pontiffs have retained the same such legislation governing Conclaves.
Also, the legitimacy of the election of the Roman Pontiff is a
dogmatic fact. See also about the 6th paragraph in section 11 of
this commentary from the CDF.
Furthermore,
the local Roman Church is indefectible as is the Church as a whole so it would not follow or accept a false Pope and neither would the entire Church if some Roman Pontiff were to tacitly abdicate due to
pertinacious heresy (and there is debate if it is even possible for him to do so)–and again, this relates back to the reason why the identity of the true Successor of Peter is a dogmatic fact.
Thanks for the info.