I doubt that. Having the funds jointly controlled by the exchange and the user, and outside the full control of the exchange, would require each fund transfer t

I doubt that. Having the funds jointly controlled by the exchange and the user, and outside the full control of the exchange, would require each fund transfer to be represented as a transaction on the Bitcoin network, which is simply impossible - the volume of transactions on an exchange is a few magnitudes larger than what Bitcoin can currently handle. To keep up with that volume, exchanges must use an internal off-chain accounting system, which precludes the use of multi-signature (or any other smart contract based on Bitcoin scripting). In addition, this would require the user to be online and actively sign transactions when a matching order is found. I think that they most likely meant that they're using multisig internally to protect their funds, in a setup where all the keys are controlled by them.