Whenever there is a conflict between Cicero and other sources, don't trust Cicero.
In this case I agree Cicero is wrong, but there are a lot of subtleties here that we wouldn't have but for this section of "On The Nature of the Gods" and now also this section XIII of Tusculun Disputations.
If we add this section of "Tusculun Disputations," to "On The nature of the gods" and also add in some of Cicero's argument in "On Ends" that Joshua references in this episode (that Epicurus identified justice with what people think is justice) I think we can get to see that there's a pattern on this particular issue that Cicero either did not understand or intentionally misrepresented Epicurus.
I doubt Cicero could have made this up from nothing, and Cicero's giving us more to work with than we have solely through Diogenes Laertius, but he's either intentionally or negligently making it sound like Epicurus took the position that if enough people believe something then that is proof that it's true.
(This calls to mind DeWitt's statement that he believed Cicero was intentionally malicious because Cicero could not have misrepresented Epicurus so effectively had Cicero not understood Epicurus so well. If I were arguing against that, I might point out that Cicero had no problem stating that he disagreed with Epicurus in highly charged moral judgments, so did Cicero really need to mis-state the Epicurean position on prolepsis? It wouldn't have changed his mind if he decided at some point that he did misunderstand prolepsis, but I can see the possibility of confusion, since Cicero did like the "common consent of mankind" argument that others appear to have been making.)
If we think Epicurus was intelligent - which we do - then Epicurus' argument can't be as superficial as saying that something is true because some number of people believe it. But Cicero has spread this allegation and it seems to have become embedded into what is accepted that Epicurus thought. I saw a Greg Sadler video in which he seemed to be accepting that Epicurus took the position that "the common consent of mankind" was Epicurus' proof that gods exist, and it undermines the credibility of anyone to think that they argued that truth can be decided by majority vote.
So defenders of Epicurus have to be able to articulate how what Cicero wrote is wrong, and be able to articulate what Epicurus was really saying instead.
It does seem like Epicurus was basing his argument for existence of gods on prolepsis, but that's not at all the same thing as saying "gods exist because everyone thinks they do." What follows after that has to be a concise explanation of how prolepses are not opinions, but are somehow an input into formation of opinions.