In some cases a chargeback may get reversed even if you have not defended it, and its status will be shown as "Won" in the Customer Area. This indicates that the chargeback was withdrawn by the shopper - they are no longer disputing the transaction. You remain in possession of the funds, and can consider the case closed.
A chargeback that was withdrawn by the shopper may still appear defendable, and you may be able to upload defense documents. However, there is no need to do so, since you have already won the dispute.