I don't think it is really possible for a mixer to not store all their coin in their hot wallet. If CM used cold wallets, moving coin to/from the cold wallet really couldn't be done on-chain, otherwise, it would be clear which groups of addresses belong to CM. Also, CM's operational model effectively put all their coin on a hot wallet, regardless of the location of it's private keys due to the fact that CM had such a low threshold for someone to get a "chip", which is effectively an IOU from CM to their customers -- all that is needed is a browser session.