That is an interesting point, and I think there are at least two factors in this tribalism. Firstly, there is a faction of the population (or maybe most of us, at some time or other) who choose what they believe on the basis of whether it would lead to outcomes they would prefer, rather than on evidence for or against the proposition. Then there is another faction who assume that if you do not agree with them, then you must have bad motives. The sort of tribalism we are seeing here (and elsewhere) arises in the intersection of these tendencies, and people who take the first position over this matter are 'primed' to also take the second: they want the virus defeated, so if you are skeptical of this treatment, you must want the pandemic to continue, right? And people who realize that the evidence is not good may suspect those who insist that the treatment is effective of having ulterior motives.