Outside having deficiencies, there's still no significant basis to believe dietary supplements such as zinc or vit D are any use, nor any convincing evidence for hydroxychloroquime. That study on vitamin D, magnesium and B12 is an extremely weak study: it's very small, and even the limited information available accepts considerable disparities in things like age between the groups studied.
The stupid thing is, at least one seemingly useful candidate for treating covid-19 has been discovered: an old, well established and commonly used corticosteroid called dexamethasone, at a cost of about £50 per patient (UK; albeit plus the very considerable ICU costs). A large and well controlled study in the UK involving thousands of patients found it reduced mortality by about a third.