Attacking the arguer's credibility isn't fallacious if their argument depends in any way on their credibility, as it clearly does in this Newsweek story.

I'm not familiar with that exception in logical thinking. What argument is logically dependent on someone's credibility?

For instance, this Newsweek article and the claims made by the doctor in it depend on the doctor's own reports of their interpretations of MRI scans. Observing that the doctor making those interpretations is disreputable is an important and logically meaningful point.

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".

It's not necessary to drag one's assessment of their credibility into this at all, though. Points 1-3 of the original post are all that need to be argued.

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.

Argument from authority. A said X, A is an authority on X, Therefore, X is true.

This is a valid inductive argument. It is valid when A is an actual authority, when they are speaking truthfully, etc. It doesn't guarantee the truth of the conclusion but it is a fine argument. How do you know Booth shot Lincoln? You read it in a textbook, the textbook is written by someone who is an authority on history or that part f history, etc. It isn't deductive (like a proof in geometry or logic) but it is a good argument. Many logic books call argument from authority invalid and what they mean is that it isn't deductively valid but it is an inductively valid argument form (If it were always invalid, we would be quite mired in ignorance).

Other people have made good replies, but this is also a pet peeve of mine, so I'm going to jump in as well.

Another way to think about it is as witness testimony.

When you do a logical, mathematical proof, you are building on a well-defined and well-known set of axioms, using well-defined steps of logical inference. With a few well-known exceptions, we don't argue over mathematical proofs. Either the logic flows from the axioms or it doesn't.

Once you step away from pure mathematics, you no longer have a small number of well-defined axioms or clear inference. Instead you have a lot of what amounts to witness testimony. Sometimes, you recount an observation. "X drowned and was dead for 15 minutes." "X had brain damage." "X fully recovered." It is reasonable to dispute these statements based on the credibility of the person making them.

Some of these statements may be the product of inference, which is also less clear than logical inference. For instance, "X has brain damage." might have been inferred from "An MRI of X looked like Y." and "Y indicates severe brain damage." Now you have a host of ways where the statement could be wrong, based on the reliability of the person making the observation and the inference. Did the person actually take an MRI, or were they lying about it, or did they misread it, or did they actually rely on a third-party who did one or the other? Is the inference method "Y indicates severe brain damage" valid? Did the person in question correctly apply it?

People don't like to attack the credibility of others; it seems mean and it can easily slip into elitism. But examining the credibility of someone presenting evidence is absolutely critical to truth-finding. Personal integrity is an absolute necessity, in science, medicine, and law.

In pure logic, credentials don't matter. However we're discussing something that is not pure logic (i.e. we are relying on the author's expertise and insight, and the publication to accurately represent that), so credibility is certainly relevant.