just my happence: I don’t think this is that fallacy. The definition of orthodox in this case is that someone (or a shul) follows the views of competent orthodox Rabbis, not specifically halacha. In this case the argument is whether certain Rabbis fall into the category of “competent”.
I don’t know who Avi Weiss is but my opinion is that if zahavasdad says an individual who isn’t Chareidi isn’t a Talmid Chacham then I’m comfortable believing that to be true.