I did have saved passwords in Chrome password manager but they were old. My guess is that the attacker used Google SSO on Coinbase (e.g., "sign in with Google"), which I have used in the past. And then they opened up Google's Authenticator app, signed in as me, and got the auth code for Coinbase.

By enabling cloud-sync, Google has created a massive security vulnerability for the entire industry. A developer can't be certain that auth codes are a true 2nd factor, if the account email is @gmail.com for a given user because that user might be using Google's Authenticator app.