True, but all the proof the prosecution has to provide is that the documents are marked as classified, and that there is no record of them being declassified.
No, that is not all the proof they have to provide.
The government must provide sufficient proof to convince a jury that the defendant is guilty beyond a reasonable doubt. The President being able to declassify any document he pleases, without consulting or notifying anyone, means that there is obvious cause for reasonable doubt even if you find an ex-President possesses a document with classified markings.