Easy. They used a mixing or what you called a tumbler services. So I think the criminals would used one of those services then mixed it there before transferring it to their wallet. They will used not 1 mixer but a lot of them and will probably delay the coins before going in their wallet. And for added anonymity, they might have moved it from bitcoin to Monero, because its more anonymous than bitcoin. They can even used the localbitcoin trading scheme as well. There are ways to really hide your self and not be track in blockchain if you know what you are doing.