Universi Dominici Gregis does not say that only 120 cardinals may vote in an election. Rather, it says that “[t]he maximum number of Cardinal electors must not exceed one hundred and twenty” (¶ 33), and then further provides: “No Cardinal elector can be excluded from active or passive voice in the election of the Supreme Pontiff, for any reason or pretext . . .” (¶ 35).
When a pope creates more than 120 cardinal electors, he has de facto dispensed with the obligation of paragraph 33. That is the “indult” I meant. The very fact of appointing a cardinal elector in contravention of the numerical limit stated in Universi Dominici Gregis constitutes a waiver of that limit. And all cardinal electors, under paragraph 35, are all entitled to vote.