The parent poster talked about an offline backup. That is the big idea of offline (and even better, offsite) backups, that you cannot delete or destroy them from a computer, just by destroying the physical media.
An ex admin would probably have access to the physical backups too, this could have been done before he left the company.
If he was still a full employee when he wiped the servers, then theoretically yes. It rather sounded to me that he wiped the machines after he left. And off-site backups are often handled by storage companies, making it more difficult to access them.