It says this in the article:

>As alleged in the supporting affidavit, by reviewing the Bitcoin public ledger, law enforcement was able to track multiple transfers of bitcoin and identify that approximately 63.7 bitcoins, representing the proceeds of the victim’s ransom payment, had been transferred to a specific address

That seems to imply that they didn't use a mixer.

Honest question: what would a backdoored mixer look like? If it had a list of trapdoor addresses (or checked addresses in real-time) and made last-minute transaction changes, say. Would any criminal risk complaining if it identified them? Does the tumbler’s reputation have a mechanism for angry criminal user stories?

The tumblers seem like a centralized chokepoint for criminals trying to launder.