People are concerned about taharas hamishpacha issues that (may) arise from the vaccine, as well as fertility issues. Besides for dealing with such shailos, SCB is also considered a non-political Rov with no agendas and bias, as well as a mench (a bit uncommon in the rabbonus).
My only interaction with the rabbi was back when I was in yeshiva, and indeed he seemed like a talmid chacham of impeccable character, a most eidel, respectable mentch.
Intresting that those worried about the taharas hamishpacha implications of the vaccine weren't that concerned about the range implications from covid, which, anecdotally, are equal or worse than the vaccine effects.
And the fertility stuff is not based on anything, as discussed in other threads. What is noteworthy is that mumps carries fertility risks as well, and the anti vaxxers were loudly silent about that concern during the mumps outbreak a few years back.
Be all that as it may, what is to be gained from being on a panel with presentations made by a cavalcade of clowns without presentations from the other side of the debate? These presenters are selected and paid for by people with a clearly stated agenda, it's hardly an unbiased look at the facts for the benefit of the community.
It seems clear that either R' SCB (and perhaps others) agree with the other panelists in their anti vax stance, and were there to have their priors confirmed, or they were convinced into joining to give the presentation a veneer of respectability.