Actually, it’s my bad—I found your comment via the new-comments list, and didn’t look very closely at its context.
As to your actual question: Being told that someone has evidence of something is, if they’re trustworthy, not just evidence of the thing, but also evidence of what other evidence exists. For example, in my scenario with gwern’s prank, before I’ve seen gwern’s web page, I expect that if I look the mentioned drug up in other places, I’ll also see evidence that it’s awesome. If I actually go look the drug up and find out that it’s no better than placebo in any situation, that’s also surprising new information that changes my beliefs—the same change that seeing gwern’s “April Fools” message would cause, in fact, so when I do see that message, it doesn’t surprise me or change my opinion of the drug.
In your scenario, I trust Merck’s spokesperson much less than I trust gwern, so I don’t end up with nearly so strong of a belief that third parties will agree that the drug is a good one—looking it up and finding out that it has dangerous side effects wouldn’t be surprising, so I should take the chance of that into account to begin with, even if the Merck spokesperson doesn’t mention it. This habit of keeping possible information from third parties (or information that could be discovered in other ways besides talking to third parties, but that the person you’re speaking to wouldn’t tell you even if they’d discovered it) into account when talking to untrustworthy people is the intended lesson of the original post.
Actually, it’s my bad—I found your comment via the new-comments list, and didn’t look very closely at its context.
As to your actual question: Being told that someone has evidence of something is, if they’re trustworthy, not just evidence of the thing, but also evidence of what other evidence exists. For example, in my scenario with gwern’s prank, before I’ve seen gwern’s web page, I expect that if I look the mentioned drug up in other places, I’ll also see evidence that it’s awesome. If I actually go look the drug up and find out that it’s no better than placebo in any situation, that’s also surprising new information that changes my beliefs—the same change that seeing gwern’s “April Fools” message would cause, in fact, so when I do see that message, it doesn’t surprise me or change my opinion of the drug.
In your scenario, I trust Merck’s spokesperson much less than I trust gwern, so I don’t end up with nearly so strong of a belief that third parties will agree that the drug is a good one—looking it up and finding out that it has dangerous side effects wouldn’t be surprising, so I should take the chance of that into account to begin with, even if the Merck spokesperson doesn’t mention it. This habit of keeping possible information from third parties (or information that could be discovered in other ways besides talking to third parties, but that the person you’re speaking to wouldn’t tell you even if they’d discovered it) into account when talking to untrustworthy people is the intended lesson of the original post.