Ivermectin is useful for control of parasites in humans (1) and other animals such as horses, but the grandparent is also correct, people are "literally buying and ingesting products intended for livestock" - from the veterinary aisle (2), and self-administering random doses not safe for humans, with bad consequences (3). It's sheer lunacy to do that. If it was effective for COVID-19 (a virus, not parasite), then medical staff would be administering correct doses.
Ivermectin is not just for control of parasites, the anti-viral properties it has is because it's a protease inhibitor. It prevents certain viruses from binding to cells.
How effective it is at inhibiting binding is the question. This pill in the article and Pfizer's prophylactic are most likely more effective since it's not a by product but the intent. Also more profitable because of patents.
1) https://www.who.int/tdr/news/2018/moxidectin-approved-as-tre...
2) https://www.iflscience.com/health-and-medicine/store-only-le...
3) https://www.npr.org/sections/coronavirus-live-updates/2021/0...