The basic immorality/pointlessness of the war on drugs aside, I don't know how you can assert this: it's not like there's a chain of provenance, and there's no particular guarantee that whatever grade of pure drugs was sold on Silk Road is the same purity that ended up in peoples' bodies.
My understanding of the Silk Road case is that, at its peak, it was servicing a significant portion of the international drug market. The dimensions of that market include adulteration; Silk Road almost certainly didn't change that.
Their observation was that reputation mattered on SR a lot and a well-kept reputation was valuable at scale in a way that it isn't for being a street-corner pusher looking to stretch your buck by cutting your supply with adulterants. The smart play was to provide a high-quality product at a reasonable price (the latter being the easiest part since they were bypassing the obscene markup of official channels).