Based on personal experience, the technical personnel who generated the data DID mark it appropriately. Therefore, one or more of Hillary’s staff members are guilty of removing the identifying marks.
But if she personally sent something sensitive she made from scratch. she could have left those markings off, as well. Heck, you don’t need to mark something as confidential, you are expected to either not send it or send it via an encrypted method (from knowledge at a different federal agency).