To understand this, you may need to understand briefly the difficult situation in Israel with regards to the Holocaust. The Eichmann trial took place amidst a broader public reckoning in Israel. In part, this is because of what /u/commiespaceinvader writes in their excellent answer here about the Judenräte.

In another part, however, the reason is because of the national ethos of Israel, and how it contrasted with the view of Jews who died in and survived the Holocaust. It's a very complex topic, but to put it in simple terms, the Israeli public had been built on a view of the tough, empowered Jew. There was a self of rugged self-reliance, as well as a determination particularly after the Holocaust to be capable of self-defense and resistance to the end. In part, this was exemplified by the "heroification" of Josef Trumpeldor, who died defending a Jewish village during Arab riots in 1920. Trumpeldor is said to have exclaimed "It is good to die for our country" before he died, and Tel Hai became a rallying cry for Jewish resistance of a sort similar to Masada.

Particularly after the Holocaust, there was a sense of motivation in the 1948 Arab-Israeli War that amplified this sense of self-reliance. As Benny Morris put it in Righteous Victims, "[n]o national collective was more self-reliant or motivated, the Holocaust having convincingly demonstrated that there was no relying for survival on anyone else."

This contrast extended, as a result, to how Israel viewed Holocaust survivors and diaspora Jews. In Bitter Reckoning, Dan Porat also talks about it himself, though in terms a bit more simplistic than I think I'd use. He says, for example, that it was a dominant trend in Israeli culture in the 1950s to "accuse diaspora Jews of 'going like sheep to the slaughter' and taking part in their own annihilation".

This context is important because it helps explain the reason Porat gives for the offer to have Jews testify on Eichmann's behalf. Israeli society had, as part of its ethos, come to view Holocaust victims with a sense of sympathy but also a sense of derision, of superiority to them. It was Eichmann's trial that provided a sense of historical and cultural importance, and a chance to update this understanding (especially now that Israel was more than a decade out from the 1948 war, albeit not out of a sense of existential danger surrounded as it was by enemy states).

To wax slightly longer on this importance, it was something Israeli leaders understood and wanted to illustrate. Ben-Gurion, Israel's first Prime Minister, put it this way, saying:

It is not the punishment that is the main thing here, but the fact that the trial is taking place, and is taking place in Jerusalem.

He continued to say, as Tom Segev recounts in The Seventh Million, that Ben-Gurion pointed out that it was not only Eichmann and his deeds but the entire Holocaust would come before the court. Ben-Gurion's goals were, (1) to remind the countries of the world of the Holocaust and obligations to Israel that resulted from it; and (2) to impress the lessons of the Holocaust on Israel, especially the younger generation. Gideon Hausner, the Israeli attorney general who did much of the trial work, knew he wanted to go beyond documents, and wanted "people in Israel and the world to come closer, through the trial, to this great catastrophe". He put it exceptionally well, explaining the cultural derision aspect as well and where it derived from (pointing especially to the fact that, because so few survived and so many lost entire families, knowledge was missing) and how he hoped to change it:

This is a generation with no grandfathers and grandmothers. It does not understand what happened, because it has not gone into the facts. The gap between the generations has turned into a chasm, creating repugnance for the nation's past. "How did they allow themselves to be led like lambs to the slaughter?" is the common question...We need a massive living recreation of this national and humanitarian disaster...I wanted [the witnesses] to talk about the various stages of the extermination from the beginning, about the large Jewish cities and what had happened to them, about the communities and people who tried to resist the disaster, and about the extermination camps themselves. More than anything else, I wanted people to report what they had seen with their own eyes and experienced with their own flesh.

Herein lies the problem and the crucial conundrum. There was an obvious division in Israel that had to be addressed, over the actions of Jews like those in the Judenräte, compared with the glorification of Jewish resistance in the Warsaw Ghetto Uprising, for example. This was the fear of Hausner, who worried that the defense would point to Jews having helped the Nazis or collaborated with them, to provide a defense to Eichmann. Many survivors continued to, additionally, harbor resentment towards those they viewed as collaborators, a complex subject given the situation of those accused and how difficult it was. The trial strategy thus was one where Hausner feared that Eichmann's defense would bring up examples of Jews "just following orders" to bolster a similar defense to Eichmann's claims of just following orders, rather than demonstrating his actual malice and evil actions. It was not necessarily in service of Eichmann, then, that those survivors were willing to testify for the defense from what I have seen; it was because they were still understandably upset and distraught over Jewish collaborators with the Nazis via the Judenräte and functionaries (Kapos) in the camps especially. It is for this reason that Porat writes that following the defense's decision to turn down the Holocaust survivors' testimony to "expos[e] Jews who they believed had acted in an immoral manner", Hausner felt some of the fear about diverted attention from Eichmann to have passed. Still, Hausner worried that the defense would divert attention from Eichmann to Jews who had worked with him, sometimes in corrupt manner, to save their own rather than other Jews. The prominent example of this that had been litigated in 1954, and which the Eichmann prosecutors did not want to relive, was the case of Hungarian Jew Rudolf Kastner, who was accused of collaborating with the Nazis (and in 1957, was assassinated as a result by far-right Jews as an act of vengeance). While a district court had eventually ruled that Kastner did indeed collaborate with the Nazis and sold his soul, the Israeli Supreme Court eventually overturned much of the decision, leaving the moral points for history to judge, broadly speaking.

There were no shortage of Jews thus upset about Jewish "collaboration", and many were as mad about those collaborators as the Nazis themselves, viewing them as traitors as well. This division, as I said, informs the decision of some to go so far as to testify for Eichmann's defense to have their voices heard. After all, Eichmann's trial was the event of its time; there was no television broadly in Israel, but the trial was broadcast live on radio, schools canceled classes to let students listen, broadcasts were translated and rebroadcast into even Yiddish, and the broadcast reached virtually every Jew in Israel, to say nothing of those abroad following events. It is also of note that some of those who offered to testify in defense may have been motivated by an opposition to the death penalty, which was almost certain to be given. Jewish scholars had, in fact, even organized to this effect, seeking to pressure Israel's leaders not to allow the death penalty for Eichmann. Eichmann's trial had the desired effect, in terms of changing Israeli culture to be more understanding of the Holocaust survivors' plight, though the Judenräte were virtually never mentioned. The gap in generations had been closed. In part, it was Hannah Arendt's own dispatches described in the answer I referenced above that did it: Arendt blamed both Zionists and the Judenräte, in part, for the Nazis' success. By tying the two together, Zionist Jews and Israel responded viscerally to both, rather than just one. This helped spur the Israeli change in tone on the Judenräte culturally, at least, albeit slowly.

I hope this helps explain the context and possibility of survivors testifying for the defense; it's hard to find the exact source that Porat himself cites, as it would require me finding a book in Hebrew (which, while I could read it, is not easily available). The other source is from the trial transcripts themselves, which are not digitized, but I believe that relates to his discussion of Zuckerman's testimony, not to the offer of others' testimony.