Offline storage just means having a physical copy of the wallet keys stored somewhere safe and no digital copies (or a hardware wallet that is equally well-hidden) that can be accessed by the network. The record of any nefarious transaction will always exist on the blockchain for all to see.

To get away with it, he would have had to deny all knowledge of the keys -- which he could potentially do with offline storage, at the expense of opening himself to perjury if he ever got busted -- but, importantly, never spend the contents in a way that could be linked back to him by following the metadata "paper trail". That last one is pretty difficult to achieve, especially when so many coins are involved and they're already "hot". A mixer could help obfuscate the trail -- especially if there were steps involving exchanging to other cryptocurrencies or fiats, but then trust becomes a real issue -- but given the current exchange rate, it would probably be worthwhile for the Feds to do some blockchain forensics to keep track of it all.

I don't think he would have ever got away with it.