You've misunderstood the correlation-causation complaint. We have a pretty clear theory of causation here and the results back it up. Just because causality depends on specific conditions doesn't stop it being causal. Any medical treatment that isn't 100% effective (ie, most of them) depend on specific conditions being present - otherwise they'd be perfectly effective. Of course since the chain of causality is quite clear on this one we can conclude from the base rates of parasite infections there isn't much point taking ivermectin for COVID in the west.
There are zero new info in studies like that (unless you believe every logical conclusion requires a new study? If A is proved, should we separately prove A or B or what?), and they can’t even shut up all the idiots that still go on about ivermectin, so not even that goal is achieved..
What the actual studies show is simply a correlation, and we can do some educated guesses based on prior knowledge: ivermectin is an effective dewormer, the human body can produce anti-bodies against COVID, and that the immunesystem is better fighting a single thing, than multiple ones. Putting these together, we get a reasonable hypothesis showing a third variable that explains the measured correlation, through a causative mechanism (parasite hindering healing and parasite getting killed).