The reality of expert witnesses is not that they are authoritative sources outside the courtroom, but witnesses for one party. It's a job - sometimes for people who didn't find much success in their field - and they are paid by the party that calls them to testify.
Everything in a US courtroom is adversarial; every witness is cross-examined and their credibility can be questioned.
>witnesses for one party
>every witness is cross-examined and their credibility can be questioned
I think we're talking about different stages. The determination of his qualifications happens before he can be a witness for one party. I'm talking about voir dire. Before we even get him on the stand, there's a separate hearing to determine if he can even be an expert witness, and the opposing party can challenge.
Anyway, there is no ongoing action that I can find. It's only just come into effect, so that's to be expected. As far as an authoritative source for the argument: the person we're discussing is actually the author.