I dont know completely how the wallets and addresses are controlled. But just a thought of me.

If I was using blockchain.info , then I can download my private key to import it to my offline wallet. I dont understand the term - "BitStamp no more controls that address" - Either the attacker stole the bitcoins by inserting malicious code or doing some sql or php query stuffs or getting access to the wallet. Even if he have the private key to that address, still the bitstamp will also have the private key with them unless the hacker gets physical access to their computer or whatever and destroys everything. Please correct me if I'm wrong.