The article didn't seem to mean in terms of the quality within a drug type. It looks like is more about the value density of a drug. So if Amsterdam cracked down on Marijuana, then there would be more potent cocaine, crack, meth, etc. Even the strongest marijuana pales in comparison to the 'potency' of the hard drugs.
...And of course Amsterdam is also serving much of the world (pretty much exclusively, until really recently) in terms of breeding new strains. The demand for high strength was global, even though the nexus was small, and in a legal haven.
