What is more surprising in this news is not that the government was able to trace these transactions even after the police officer's efforts to eliminate the trace (I suppose that he made a mistake at some point) but the fact that the delinquent was a policeman.
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.