Why would they be stupid to hold the bitcoin themselves? If they don't hold the private keys, then whoever does could lose them.
By hold, I mean own. Holding as in holding the actual keys would be another level of crazy on top of that.
If they hold the Bitcoin themselves, they have to deal with securely holding the keys, compliance, and insurance. If someone else holds them, then this third party has all of these issues while GS will still get some kind of insurance payout if there is a catastrophic loss.
Similarly, I doubt that GS commodity traders have warehouses full of gold/silver/oil.