What they're getting confused with is that as the craze hit and Ivermectin supplies ran low, some people were buying it from veterinary supply stores in formulations meant for horses.
People were, quite rightly, saying that taking medicine formulations meant for horses was a bad idea, given the potentially non-human-friendly additional ingredients and the difficulty in keeping within safe dosages.
It's for worms. If you have a persistent infection of parasitic horses, it's probably not going to do much good.