A substance can have pharmacological effects and still not be recommended for therapeutic use. As a hyperbolic example, suppose a substance relieved all pain for 1% of the population but caused death in everyone else. Even with a highly precise screening process this substance likely would not be administered in medicinal contexts.

That's true, but I believe the authors' complaint here is efficacy rather than safety. (I also think they're using terms of art from evidence-based medicine to make a statement the general public is likely to misinterpret, per my other comment here.)

Safety is barely discussed in this paper, probably because the available RCT evidence is favorable to cannabis. I'm not sure that means it's actually safe, since RCTs of tobacco cigarettes over the same study periods probably wouldn't show signal either. This again shows the downside of ignoring all scientific knowledge except RCT outcomes, just in the other direction.