Of course they weren't mixed on Dash's blockchain, because Masternode Blinding is still a 'work in progress' or whatever.
Mixing 450k Dash-coins with Darksend would take at minimum 2^7688 years. The last quark would decay into strings and branes eons before they finished.
The only alternative is using cross-chain mixing via different exchanges, which is exactly what your post illustrates.
Any proof the coins went to different owners, or are you just assuming the hackers didn't sell to themselves as a cross-chain mixing/plausible deniability scheme?
I can only show what the blockchain shows, that they were slowly sent to various exchanges... after that I can't say for certain...
I assume they were sold because many of the addresses are marked as "cryptsy", "GHash.IO", "Poloniex", "Bitfinex", etc... which typically happens when a person withdrawals and marks the wallet as owned by the exchange... so I assume at least some of the coins were sold and withdrawn by other users who marked said wallets on cryptoid.info
But, you do have a point that some of them might have just been mixed and not sold (unlikely given the number of marked wallets)...
Who knows for sure? Do you have any proof that they were
not sold? Any evidence I can find points to all 456k being sold by Nov 19, 2015