I also don't understand that position. If somebody doing research in a certain domain comes to talk about that domain, then whatever they say will be relevant. Even if ill-intentioned, they will know their stuff and come up with a valid critique which needs or needs not to be taken into consideration. Now if a neuro surgeon - even if genial and successful - talks about vaccines, there's already a serious chance the topic goes astray, and the further a person removed from field is the lower the trustworthiness of their comment. I know, there are exceptions to this rule, and everyone thinks they are that exception...