The only other way this could possibly be done is if the redirection thread was created by that user and either theymos had a stroke, intentionally moved the topic over to be owned by anohter user, there was a database error, that user used some sort of sql injection in order to bypass a database security section and change the ownership of a thread temporarily to themselves and give it back after they had done the change or that account is theymos' account.
This scenario painted is very scary and places a huge security burden on the forum. I hope this isn't what happened. Only moderators or empowered few have the enabled feature for deleting, moving or moderating posts. I know this because I am a mod on another site. Even as a mod you can't even moderate sections not assign to you.