Laundering within the constraints of a public ledger isn't feasible for long periods of time or large amounts of money - the only way to win that game is to be so small nobody cares.
They could have possibly gotten cash from cartels at a steep discount, but that story would probably have ended with a richer cartel and two dead nerds.
I always wonder how many people won this long con by being so small nobody cares. The DoJ document states they already succeeded in taking the funds cross chain and through some privacy enhancing alternative assets. For every idiot dumping millions in a bank account there's got to be someone else living a "modest" but luxurious life looking like a small guy nobody cares about, cashing out a few hundred to a thousand at a time somewhere where that kind of money is big enough to get a nice day to day living but small enough to not be worth organized crime taking much notice.
On the flip side, most of us could already live quiet, comfortable lives in Thailand or Cambodia on our normal software engineering income.
Maybe these guys are thinking if you're going to be a criminal, you had better make the juice worth the squeeze. Live large, party extravagantly, and probably just rent everything from hotel rooms to yachts so there's less for the authorities to eventually impound when it catches up to you.
They could have possibly gotten cash from cartels at a steep discount, but that story would probably have ended with a richer cartel and two dead nerds.