It's technically true for another reason as well. The word 'classified' is not a classification, such as 'top secret,' so no classified document is marked "classified." Classified refers to a general category of documents that have various classifications. Like 'SAP.'
Here is a government-produced guide to classified document markings from 2001. The chapter on Markings starts on page 17 with examples of classified documents shown.
Guide to Marking Classified Documents
It's technically true for another reason as well. The word 'classified' is not a classification, such as 'top secret,' so no classified document is marked "classified." Classified refers to a general category of documents that have various classifications. Like 'SAP.'That is correct. Someone needs to ask her: "Did you ever send or receive emails specifically marked "Confidential", "Secret", or "Top Secret?". As you've stated, there are no secure documents marked "Classified" (except maybe in the movies or on TV.) This is Hillary's version of "it depends on what the meaning of the word "is" is."