I'll elaborate on why this matters, speaking as someone who works in IT for a fusion lab and has to deal with securing physical and electronic nuclear secrets on a regular basis:
1) There is a legal obligation to anyone aware of a physical document marked in a way that identifies it as classified and not possessed of the proper clearances (or unsure whether they have the appropriate clearances) to report the presence of that document to a proper authority (which varies depending on the type of document). In many industries, you cannot be employed without receiving considerable training on this point, including the legal consequences that may pertain to anyone who fails to report. (Consequences may vary depending on several factors.)
2) A similar responsibility pertains to anyone reading a document that contains information that should be explicitly classified but has not been marked as such. (You can still be imprisoned for passing on a file that isn't marked "Top Secret" if it should have been, and the only defense against that is if the material is secret enough that you had no way of even knowing that it should have been secret. Even then, you are trained that it's far better to assume that any document discussing certain material should be classified, even if unmarked, and report.)
3) It is the legal responsibility of the person receiving the report to verify that, based on what they've been told, they have the proper authority to validate the appropriate physical security of the document and ensure that it is not seen by other people with insufficient access. If they don't have proper authority, it is their job to escalate.
4) A security audit has to follow, ensuring that anyone who has seen or may have seen the document is questioned and investigated. The document itself is secured and its contents may be considered compromised. If it was improperly unlabeled, it will be labeled; if it was improperly secured, it will be secured. "Improper" in both those cases is defined by statute. All actions taken are recorded in the audit log associated with this document. (If the document is being correctly classified for the first time, that will be noted in the audit log; otherwise, a log will already exist.)
5) If the document is re- or declassified, it is physically marked and recoded to reflect its new classification. This event is recorded in the audit log.
6) If the president decides to physically hand someone a classified document and retroactively decides to declassify it, both the improper handover and the declassification decision are noted in the audit log, and the document is modified to note its declassification.
You'll notice that at no point is there a legal out for "but a former president said he declassified this some time in the past." You don't, as an agent of the government, get to use that excuse. You are legally mandated on penalty of prison time to follow these procedures and, equally importantly, ensure that they are followed. While certain executives above you in the chain of command have more discretion, notably one of the executives not included in that list is any former federal employee.
