First I'm willing to bet that they made 100% sure they can sign a tx out of that new address. Even if the address is seen for the first time on a public ledger, they may very well have tested first that the HSM allowing to sign a tx for that address was working on a private, offline, chain.

Then the transaction itself to move those 6 bn has probably been prepared offline, on a fully airgapped computer, signed with a HSM and then later one broadcasted (once it's been carefully inspected).

The amount is 6 billions: I think that the precautions taken here are beyond many's imagination.