To be honest, an email is really the least of your worries. Yes of course they make the recovery process easier but if someone is the original owner of an account and you purchase it from them. Even if you have your own email and recovs on it, they'll be able to recover it if they are the original owner.
However to answer your own question, it means they were subscribed to Jagex with an email registered to that account but at some point in time that email link was removed.