The medical profession is extremely conservative when it comes to treatments because we take our oath (Above all do no harm) extremely seriously. Every drug has a side effect, and we need to weigh that against the possible treatment. We do not just start treating people based on one study, we only do that after multiple studies show the same effect. Right now plaqunil has 2 studies suggesting it might be effective but multiple that show no effect and others which have been stopped prematurely which (while I can’t say for sure as they havent been published) almost certainly means they saw a negative outcome (increased morbidity and/or mortality.). The FDA was so convinced that this drug could harm people after seeing that data (They get that access) that they stopped the ability of doctors to use this drug.
Does that mean the new study is wrong? Maybe, there is a chance that they saw a coralllation that isn’t true because their sample was not representative of the whole population (which can happen even if you do everything right.). The only way to prove the study is right or wrong is to have other groups attempt to replicate that study elsewhere, if their results are reliable then that will change the concensus, but that takes time.
As of now the FDA feels that it is not appropriate to use hydroxychorolquine as treatment.