Sure, it's not generically true, but PhilGoetz is thinking about a specific application in which he claims that it is justified to regard the expert estimates as independent (conditional on Q, of course). I don't know enough about the relevant domain to assess his claim, but I'm willing to take him at his word.

I was just responding to your claim that the detectors must suck if the correlation is small. That would be true if the unconditional correlation were small, but its not true if the correlation is small conditional on Q.