I do not think this argument is relevant to estimating effect sizes, rather than binary outcomes as the Less Wrong post does. If the claim had been "no patient

I do not think this argument is relevant to estimating effect sizes, rather than binary outcomes as the Less Wrong post does. If the claim had been "no patients treated hydroxychloroquine recover", and the study found no patients that recovered, the argument would apply: there's no evidence that patients recover, which starts to suggest that indeed, patients do not recover. But here we are interested in estimating the size of the effect, which can take any continuous value. The statistical result is that the relative risk of death is, with 95% confidence, somewhere between 0.13 and 2.89. You can't interpret that as evidence relative risk is exactly 1 (equal risk with or without the drug) -- if you could, you could also take it as evidence the relative risk is 1.2, or 2.7, or 0.75.