Bank account numbers aren't secret, they're written on the bottom of every check you write. The story lacks the details of how he got his hands on it but its not unreasonable to assume he was able to access such unprotected information.

Mitnick was exceptionally gifted at social engineering information out of various company departments over the phone.

> How would he have known the CEO's bank account number?

Welcome to the american banking system.

He used the CEO’s voice to access AN account, I don’t think it was the CEO’s specifically. But just an account, verified by the CEO’s voice, to his.