If you leave traces before using mixers or after using mixers, you are visible by governments and they will find you, that's it. Privacy and anonymity are hard to gain as it is not achieved by a single action and moment but a long history of your actions over a very long time. It's like security, as if you have a hole on a very long wall of security, it can be exploited and a whole wall can collapse.
Similarly, if you left traces somewhere, other careful actions before or later that can not help you gaining complete privacy and anonymity.
The story reminds me of the hacker, Jimmy Zhong, who stole 51680 BTC from Silk Road. After using a mixer to mix the stolen coins whenever he wanted to move them, at one point, he mistakenly sent some portion to a centralised exchange where he had KYC'ed, and that was how he was identified. I'm guessing the police officer's case is almost the same. He must have linked his identity to the address before or after using a mixer.
Whether a hacker or a thief, while doing some secret work, he will make some mistakes that will be left as evidence. He did transfer Bitcoin from Silk Road, but his brain was not so good at judging that even if he did something clever, it would not have been possible to identify him.
He accidentally transferred some of his stolen Bitcoin to a central exchange where his documents were used to verify his KYC. That is, his identity was accidentally exposed. It was a mistake made by his brain due to an unexpected reason, which people make after committing a crime.