If a statute (act of Congress, signed by the president) mere says the president can declassify documents without specifying a formal procedure, meaning they left it for the president to decide on the procedure (which would be the procedure under administrative law theory), then Trump as president could declassify by “thought” or more significantly, by physically removing the documents from the White House and taking them to his personal residence.
If the statute has a formal procedure that Trump didn’t follow, Trump doesn’t have a valid argument, since the president is subject to all statutes. However if the procedure is a matter of administrative law, the president has head of the administration can largely do anything he feels like unless it violates the constitution or a statute.
One should note that for most of the last 80 years, the Congress has tended to pass laws that tell the administration to make the rules to implement the law, and leave all the details to the administration (rather than putting the details in the statute).