The fact that people were obtaining it from sources intended for horses is mostly unrelated to the fact that it is not effective as a Covid treatment for people who don’t have some other parasite in addition to Covid.

I don’t think calling it a horse dewormer is helpful, despite the fact that people were obtaining it from people who were selling it for the treatment of horses. People who (wrongly) think it works as treatment for covid will probably be aware that it is not exclusively a treatment for horses, so I don’t think calling it a horse dewormer will change any of their minds.

Even if it did, I am uncomfortable relying on misleading people in order to convince them of something true. (If someone believes X, and X is false, and you want to convince them of a true statement Z, then it is fine to say “X is false, but, I know you believe that X. Y is true, and X and Y together clearly imply Z. Therefore you should conclude Z.” (Assuming Y is true.) That’s not misleading someone. It is something different.)

It seems like allowing oneself to mislead, even if only when the misleading statements are sorta true in a sense, just not in the way they will be interpreted, in order to convince someone of something that is true, is rather dangerous. If such behavior were universalized, I think it would be rather bad for collective understanding. (What if someone U is trying to convince someone V of some statement P which U only believe is true because someone W misled U in to believe P in order to convince U of some other statement Q that is true, and then U says something misleading to V (leading V to believe a false statement R) in order to convince V that P ? Not good.)