It is certainly not using multi-sig for their customer accounts. Maybe once they initiate a purchase, the system puts the money in escrow with a multi-sig transaction but before that, the money sits in at an address protected with a single key.
If you want to see why, follow the link they provided to blockchain.info
https://blockchain.info/address/1CatnMd3jsEKhwhSLUf8V862im8gBp3NDFThere are 4 transactions that totalled 50 BTC each. Click on any of them. They have lots of small inputs and a single output. Every input corresponds to a customer account.
Click on any of them. Look for the address in the output side. It's the transaction that funded that account. Follow that transaction. The output script looks like OP_DUP OP_HASH160 xxxxxx OP_EQUALVERIFY OP_CHECKSIG which is a standard pay-to-hash transaction.
Basically, their system has an inherent flaw. When a custom funds his account, he does a normal transaction. They have a script that collects everything from all the deposits and moves it to their own address. From there they can do the multi-sig stuff.
The developper of the website gave the tool to the hacker himself. The hacker just had to change one parameter, the target address and he was done.
Honestly, this looks like very shabby work and also shows once again that we shouldn't believe the marketing crap. Multi sig ... right