Depends on your definition of peer reviewed. The document referenced in the letter is a compilation of all the studies that have been published on the HCQ, azithromycin & zinc usage. Their peer review was looking at the evidence presented in the studies, not that the results were replicated in a separate review.
Also notable, they point out that the VA study that was widely reported last week is an outlier to the other reports & studies. Their theory is that the VA patents were already too far gone for the drugs to be effective.