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".