It doesn't really matter. If they use decentralized exchanges to convert XEM to BTC, and then BTC to XMR, they have done a pretty decent job at getting rid of their trail as much as possible.
The only possible way the hacker could break the link to the stolen coins would be via an exchange. That's the only place you can pool coins and withdraw other ones. None of the exchanges will touch these coins. They'll be frozen the moment they hit their wallets.
I don't think XEM is on any decentralised exchanges and I dunno how they work. If they pool funds, which is what the hacker needs, then that's no longer decentralised surely.
The only other way the hacker could sell would be OTC. The OTC buyer is paying for completely useless coins. It's possible the hacker could get something if the buyer is stupid and does no research. The coins themselves are effectively burnt unless they're sold back to Coincheck.