It's still not good enough to stop whoever instantly hacked my account after sending BTC to it
It doesn't look like his Coinbase account got compromised from a theif accessing the account, ... there's something else going on with this:
Since Coinbase doesn't give out private keys there's no way for a thief to create a transaction that looks like a "You sent Bitcoins to an external account" 10.399 BTC withdrawal in the Coinbase Transactions history but at the same time have a transaction combined with other inputs at the blockchain transaction level.