How would that many transfers happen in short order without the brokerage being aware? They do monitor for unusual activity. If all their assets were stolen I'd guess that would seem unusual.
There is always a false sense of security, and it has roots in our need to feel safe. That helps many of us act properly, rather than being afraid of participating.
But the pendulum always swings too far, doesn't it? Their is no single definition of security with regard to financial institutions. Oh, they have various seals that attest to just how secure they are now, but things change. The attacks evolve every minute of each day. Reminds me of a biological virus...
The protection is just devices, programming and people. Most devices just sit there and perform. The programming in the device and systems that are watching is flawed, and needs patches, updates, fixes, etc. People apply the fixes as best they can, but people are not infallible.
What if an external entity plants new code in the devices that makes it seem that transactions are normal, and nothing to worry about?
If the NSA can/has planted firmware code in hard drives, it seems to me that one attack vector in the future will be the compromise of devices that can have a catastrophic and lasting effect on the economy.
This article describes many compromises that have taken place in this century. It's filled with jargon and tech-speak, but it is not science fiction.
Your hard drives were RIDDLED with NSA SPYWARE for YEARS • The Register