I think we have a claim without a slant. You took the question to be skeptical of the loons; the claim of the loons is that helmets do not make the bicyclist safer. I took the question to be skeptical of the modern consensus; the claim of the consensus is that helmets do make the bicyclist safer. I'm not sure what the questioner is skeptical of, but citing the modern consensus that helmets make the road safer seems less reasonable.
Assume the question was, "Does water really stop dehydration?" I'd go so far as to say the only reasonable thing that could require substantiation in the wild is that water does not stop dehydration. That water does stop dehydration, even if not true, is such a ubiquitous claim it doesn't need to be evidenced in the wild.
On a second read, I understand your view too, I think if the question is ambiguous in what the questioner is skeptical of then perhaps we can substantiate both claims: it doesn't seem like it would hurt.
Either way, I fundamentally believe that a reputable claim that bike helmets do not make the rider safer is of more value than reiterating that others say bike helmets really do make the riders safer.
Certainly, on the more ape shit crazy ideas the concept of affirming the consensus seems more absurd. Take "Did the WTC really fall because of jet impact?" What would you put down for that question to substantiate the claim of it being in the wild?
"According to NIST, The United States, CNN, MSNBC, ABC, CBS, The Pentagon, and most every independent body -- yes!"