Just as there is a level of alcohol use that is "zero risk". But that level might be one drop in a glass of water at meals, and a level which has zero pleasurable effects.
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".
They drink wine on a daily basis in at least some of the blue zones where people regularly live to a hundred years old or longer. So it's not reasonable to say alcohol is relatively unsafe. Relative to what and in which circumstances?
As far as cannabis being "pretty darn safe", a recent study shows it causes heart disease. And more than one study shows it increases the risk of heart attack for up to an hour.
What we really need is an end to prohibition, and continued funding for studies on the substances that are the most popular so folks can make the most informed choices.
To apply this to your example, cannabis was linked to heart disease in daily smokers. As one would expect, the study showed a dose dependent relationship with cannabis and risk.
Just like you can find “some” people who drink daily and become centenarians, you have to be careful about the way you characterize the risk. I believe the better data show more than 4 drinks a week does increase your health risk substantially.
In both cases, it seems like moderation is key, although I maintain cannabis is far safer when you compare the ratio of therapeutic to toxic dosage. But that’s acute risk, where the study you’re talking about is chronic risk.
