The thing I don't understand is some people's implied or sometimes explicitly expressed belief that medical researchers are suppressing promising COVID treatments for some reason.

Clinical trial research is perhaps the most complex and challenging type of research to get right. It's necessary to be critical of clinical research and meta-analyses, so pointing out serious research flaws doesn't mean someone is rooting against a safe, effective, cheap COVID treatment. The VAST majority of candidate drugs don't work well even for the intended indication, let alone have serendipitous effect in a different disease.