I don't think this is true, on my account (also signed up online) I can go to the below link and click "Cancel Subscription" then choose the Use Account option instead of calling in or using their customer service chat.
edit: chat took 30 minutes from start to cancel. mostly dead air. when the chat starts they also asked for a phone number (which i never gave, so it wasnt for ID confirmation) but i instead just entered 'NA'