See below; the observation is that the people who were buying individual quantities of drugs from SR were not at serious risk of harm in the first place, relative to typical at-risk populations. Anecdotally, the people I know who bought drugs from SR during its heydey were very much test-everything-twice types.
By contrast, the large sales that SR facilitated almost certainly ended up in street drug markets, where harm reduction would have made a difference. But those people didn't benefit from SR's community standards, insofar as they existed: they got whatever adulterated product made it to them.
This is the basic error in saying "most sales were small": the big sales are what matter, socially speaking.