I remember that you mentioned underlying parasitic infections might worsen Covid-19, which may explain the efficacy of anti-parasitic drugs like ivermectin against Covid-19. While I've not read about this in-depth, it's certainly a thought-provoking hypothesis. Pre-screening in ivermectin RCTs is also a good idea to test this hypothesis.

Although randomization might control this factor to some extent, it won't tell us much about how ivermectin really stops Covid-19, as you have said.