Certainly, there must be a state for "leaked" classified documents, for which this rule simply wouldn't apply (these leaked documents could literally appear anywhere on the web in theory).
"Classified" remains classified even if leaked to remove ambiguity and possible sloppiness.
It is to avoid situations like: "this document says it needs to be handled as if it is classified but I am pretty sure it leaked last week so I will treat it as unclassified which is easier..."
It is to avoid situations like: "this document says it needs to be handled as if it is classified but I am pretty sure it leaked last week so I will treat it as unclassified which is easier..."