He could have removed himself, he’s not a random user but the CEO. I feel like that kinda removes the argument of ‘someone could have added him without his consent’.
That implies he was even aware that it happened, and again, we have no way of knowing that. If someone makes you a mod all that used to happen was that you get a message in your inbox, and he must receive thousands of messages all the time as he's being tagged in things non stop. Also he was literally a mod on thousands of subreddits, unlikely he added himself to all of them, or that he even reviewed the list.