You can dispute pretty much any payment with your payment provider. You probably don't remember having the ability to do so just because it's unlikely you were specifically looking for it. Sometimes reversals are a necessary evil for stolen cards or hacked accounts, and unfortunately, sometimes they can be fraudulent. However, there's not really anything that can be done about them. Envato can't stop chargebacks from happening, the money is taken from them automatically.
As long as it's less than six months then yes... they can dispute the charge, get their money back, and as long as they downloaded the file, they will still have access to it. If the person who downloaded the file is the same as the person disputing the payment that is.