People are really bad at accepting information that superficially contradicts something they want, and react by disputing the information instead of taking it into consideration while still pushing for the ends they want to achieve.

In this case, I'd speculate that harm reduction advocates aim is to (tautologically) reduce the (physical/health) harm of drug use so that it's one less problem users face. One aspect of this may be safe consumption, which this article says doesn't work on it's own. Another aspect is overdoses form Fentanyl et al, (which I would argue is largely a byproduct of a drug war that has made heroin too hard to import, and no safe medical supply options for treatment). So the focus could shift to addressing this (which in tandem with safe injection sites could still produce better overall outcomes).

My point is that it's not productive to dispute the studies, better to accept on balance of probabilities and look at how it fits with your overall aim.

(Climate change is very similar. People dispute the studies because they don't like the political rhetoric about how we should react, not because deep down they think all the research is flawed)