The *proof* of solvency by Mark Karpeles back in 2011 where he moved 42424242 btc could easily have been generated by collusion with a couple of whales.
You could collude to fool auditors too, right? Or do they look at the entire transaction history? That seems like a boatload of work.
If someone is standing there watching you push the keys on the keyboard while you push 500k coins across the blockchain it's a lot more convincing than a transaction that occurs the same time as an IRC conversation.
You only need to show you control the private keys.