The law enforcement authorities are able to trace back the transactions to the users, but if the mixers are used, then that makes their task a lot more complicated. And that is one of the reasons why they are trying to close down the Bitcoin mixers (or comically, trying to make KYC mandatory for all the cryptocurrency mixers, including Chip Mixer).
Do you have a source that says authorities are trying to close down mixers?
Anyway, the source itself doesn't say that their Bitcoin transactions led to their arrests, so I feel like this article is just making assumptions here. All that has been mentioned is that the suspects are known to use Bitcoin for laundering money, among many other methods. Certain addresses have been
blacklisted too, though I have no idea what implications that brings.