Calling it a horse dewormer came about because folks were buying the ones labeled as deworming agents at tractor supplies and similar locations. Sure it has other uses but its an easy jab at the folks drinking it like water for covid.

It is approved for use in humans, but its primary uses are all as an antiparisitic drug. And primarily worms.

Nobody is claiming that it isn’t great for deworming horses and other related anti-parasitic applications. But it is provably not effective at combating COVID or cancer, and mocking people who conflate the legitimate uses for the imaginary uses under the guise of “its a WHO essential medicine” is morally correct.