https://news.ycombinator.com/threads?id=throwaway7890
Credibility is not "exempt" from scrutiny, it simply doesn't change the truth value of any factual statements about whether the treatment works. If it somehow could change the truth of whether this approach worked or not, then if this guy presented the opposite conclusion, somehow this treatment could either start or stop working based only on the person's opinion and reputation.
There's obviously no causal mechanism for that to happen and believing it would require magical thinking. So arguing over it is irrelevant and inserting it into the argument is pointless. We instead can properly say that he hasn't answered a lot of good questions about his approach or presented sufficient evidence to believe in it, which is the rational approach.
Really what you're observing is that the argument in Newsweek sidesteps logic and appeals implicitly to the authority of that doctor. Presented with the argument that the subject of the article is credible enough to be believed on their own terms, we're left with a debate about that subject's credibility.
The weird idea embedded in this meta-debate is that alone among all the innumerable topics we can address with logical argument, from rankings of Led Zeppelin songs to the proper distribution of ownership of the means of production between capital and labor, the credibility of a human being on any particular subject is exempt from scrutiny, lest we fall into the trap of "ad hominem".