So if he wants to argue that he did declassified those documents while he was still President, his lawyers have to submit some shred of evidence to support that. At least an affidavit or declaration from Trump himself stating that he declassified them. But they won't do that.
One thing that is important to remember here is that if President Trump did declassify those documents, then they are declassified for everyone as of that instance. Anyone could make them public without repercussions.
If Donald Trump decided, while he was President and without telling anyone, that the documents are declassified, is he guilty of committing a federal crime?
If a criminal conviction of Trump were to be reviewed by the Supreme Court, would they rule that Congress can REQUIRE that the President face imprisonment if he doesn’t declassify according to some prescribed process?
Please keep in mind that the Constitution puts no limits on what the President can do with the secrets that he maintains. Nor does the Constitution give Congress the authority to dictate to the President how he must handle secrets.