I'm not familiar with the formal account takeover process at Google, but my best guess is that the attacker simply requested an account takeover via the official Google process, which triggered this email to be sent by Google legitimately. By reading back the code in that email, the attacker was able to claim the Google account as theirs, thus access the Gmail inbox to reset the Coinbase password and access the authenticator backups from the Google Drive.
I would be very curious to see the original message headers of the email though.