Similarly, if we really believe Ernie that the argument he gave is the best argument he could give, which includes all of the inferential steps that Ernie executed, and all of the support that Ernie took into account - citing any authorities that Ernie may have listened to himself - then we can pretty much ignore any information about Ernie's credentials.

It might take an intellectual life-time (or much more) to get all the relevant background. For example, mathematicians (and other people in very technical domains) develop very good intuitions about whether or not certain statements hold. They might be quite sure that something is true long before they are able to give even a sketchy proof and it seems rational to follow them based on their credentials (e.g. having made contributions to this sub-discipline). Yet there is probably no way to really get a grasp of their inferential steps without having done lots of the math they have.

I stress "doing" the math, rather than reading about it. Lots of math is "knowing how" rather than "knowing that". The same sort of thing might hold for aesthetical or ethical judgments. Without having played (or at least studied) a lot of classical music for the clarinet, I might not be able to grasp the "inferential" steps that led a professional player to his judgment about the superiority of a certain piece of music.