“You have no credentials or expertise to make public health recommendations” is not an ad hominem.

That's exactly ad-homimem. Instead of arguing against his points directly, they are dismissed because of who thre author is...

Whether the author is a health official who has authority to do something is irrelevant to the substance of their opinion.

The traditional ad hominem is a personal attack on the character, beliefs, motivations, or characteristics of the speaker, which is why they’re considered out of bounds in polite society.

Discussing the experience and expertise that someone has in a subject when they’re making policy prescriptions is absolutely in bounds. While it sounds high minded to say that we should argue everything on the merits alone, the brutal reality is that this results in the experts (who are few) being shouted out by the amateurs (who are many).

Expertise matters; this isn’t debate club. If you’re going to start making extraordinary claims in a field you just started studying last month, you must provide extraordinary proof. Not having expertise in an area doesn’t make you a bad person, it just means that maybe we don’t have to listen to your take.

"Ad hominem" literally means "against the man", and it refers to the logical fallacy of attacking the attributes of the man rather than the logic of the man's argument. That's all there is to it, and you're simply wrong.

Well, perhaps to avoid such attacks in future he could establish some sort of credentials in the area of interest.

Perhaps by gaining a degree and employment in the area of health and medicine?

Most educated medical professionals absolutely agree that homeopathy, Chinese medicine and chlorine necklaces do not help against the virus [except to the limited degree placebos are useful].

A software reliability engineer citing LessWrong and reddit to argue epidemiologists fundamentally misunderstand how to do epidemiology is much closer to the instinctive contrarianism of the average defender of homeopathy....