AKA "any market that sells fentanyls probably isn't being running by dutch police"

From TFA:

"During their time as black market administrators, the Dutch police only banned one product on Hansa: the highly dangerous opioid Fentanyl. All other drugs on the site continued to flow freely, a circumstance over which Ras and Boekelo seem surprisingly unconflicted. "They would have taken place anyway," says Ras without hesitation, "but on a different market." "

Exactly - the Dutch police let Fentanyl be sold freely by shutting down Hansa, where it wasn't allowed.

What? They stopped the selling of Fentanyl when they had access to the site. Hansa allowed Fentanyl to be sold before they got into the servers.

If they had continued to run the site they would reduce the amount of users of other marketplaces, where Fentanyl is sold.

Basically, if the government ran a legitimate darknet marketplace, they could reduce the sales of the worst substances.