Ivermectin is a very useful broad spectrum anti parasitic agent suitable for many mammals, not just horses, and not just against worms.
In large unvaccinated populations in, say, Brazil, where there is a siginificant worm problem in humans Ivermectin correlates well with COVID recovery - treating parasitics improves health and the ability to fight infections such as COVID.
However, as this report points out in the body, in large unvaccinated populations with no significant human worm infestation:
The ACTIV-6 and COVID-OUT placebo controlled trials of ivermectin at 400ug/kg for 3 days, and 600ug/kg for 6 days in largely vaccinated Americans, regardless of risk factors, found no clinically significant benefit in sustained recovery, hospital admissions or incidence of long COVID over 10 months
Ivermectin doesn't directly do anything for fighting COVID .. but, as one would expect, you're better off getting rid of parasites if you want to fight off a virus.