That's just it, though. The WHO says there is no level of alcohol use that is "zero risk."

The difference between toxicity and therapeutic use is what helps define the safety of the substance. There are some, like psilocybin mushrooms, that have a relatively monstrous gulf between therapeutic doses (in the mcg) and toxic doses (presumably in the kg). They are quite safe, though not zero risk. I believe alcohol is relatively unsafe by that measure, while cannabis is pretty darn safe. But that's not the same thing as saying "zero risk". I think the poster is just imprecise with their language when they mean to say "safe" and not "zero risk".