Available evidence shows some negative audience reaction to the hosts’ shift away from COVID boosters and vaccines, but not clearly on the scale of “a large number of people,” and we lack visibility into the main discussion venues (YouTube and X/Twitter comments), so the prediction can’t be firmly judged.

What we can see:

• The episode’s transcript confirms the on‑air moment: all four say they are not getting further boosters (“I’m never getting boosted again… I’m not getting boosted again now”), followed immediately by Friedberg’s remark that this will “inflame a large number of people just hearing us say this,” framing the prediction. (podscripts.co)
• PodScripts, which hosts the full transcript and allows user comments, explicitly shows “There aren’t comments yet for this episode” for E113, indicating no visible discussion there despite the tool being designed for it. (podscripts.co)
• On the fan subreddit r/TheAllinPodcasts, a June 27, 2023 thread titled “Vaccines” notes that “this sub really got agitated once their deeply held beliefs on COVID vaccines started getting challenged,” and another commenter references the hosts’ argument that COVID shots “are not a vaccine because it doesn’t prevent you from getting Covid,” closely matching language from E113. (reddit.com) This shows some audience agitation and pushback over their vaccine stance.
• A related June 18, 2023 thread, “Vaccine/Mask mandate blame,” criticizes the hosts for “rewrit[ing] history,” calls them “morons,” and disputes their claims about vaccine efficacy and mandates—again indicating negative reactions from part of the listener community. (reddit.com)
• External media pieces in mid‑2023 (e.g., KFF Health News / Genetic Literacy Project and Salon) discuss the All‑In hosts’ evolving positions on COVID vaccines and their platforming of RFK Jr., portraying them as contributing to anti‑vaccine narratives, but these articles focus on later episodes and broader behavior, not specifically on the E113 booster comments or a discrete backlash to that segment. (salon.com)

Why this is ambiguous:

• We do see clear negative reactions—Reddit threads with criticism, claims that the subreddit “got agitated,” and accusations of grifting and dishonesty. That supports some part of Friedberg’s claim (that their vaccine/booster stance angered people).
• However, these conversations involve tens of comments in a niche subreddit, not obviously a “large number” relative to an audience of hundreds of thousands per episode (E113 is listed at ~361k views on YouTube). (metapodcast.net)
• We cannot reliably access or aggregate YouTube comments and X/Twitter replies, where most of the audience reaction would occur; absence of mainstream coverage calling out this particular booster remark also suggests there was no widespread, viral backlash centered on that specific statement.

Given (1) some documented but limited backlash, (2) missing data from major platforms, and (3) the inherently vague threshold for “large number” and “strong negative reactions,” the available record does not let us confidently label the prediction as clearly right or clearly wrong. Hence, “ambiguous.”