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.
He might have linked his identity, which is a reason to think about how he was identified, but then again, he could have been very careful in covering up all his tracks but messed up by trying to use a blockchain explorer to check how everything was unlinked, which most explorers keep an IP record of and attach it to the search history, which could also be another easy way to link the user if such data is given out.
Another possible way could also be that everything is covered from the beginning, but the official was in a raid of suspects, and any point of mistake can trigger a deeper investigation in which he will be forced to confess and give out information that will make the case very easy to link all the transactions up.