From the article:

The blockchain, a ledger of every Bitcoin transaction since Bitcoin was created, is completely public.  This is hardly untraceable.  We can't trace the final receiving address until it spends some of its money.  But you can trace anyone who gave Bitcoin to the perps by following the chain back to an exchange where fiat turned to Bitcoin.  Then you find that person and track where they sent Bitcoin.  Detectives should be able to locate where the person got the BTC address they sent to and locate the owner of the address that way.  For example, someone posted the address on a forum.  Now you check the forum's records and get an IP address and you have a person.  Repeat the process and follow the chain until you get to those who paid for the show.  The actual perps may be well hidden.  But those paying for the show can't guarantee the Bitcoin they received and spent to be untraceable from exchange to them.