That is a different argument, and it's worth separating the two points. The first argument is whether documents are actually declassified by a President simply thinking it, but not telling anyone or writing it down. I think the answer to that is going to be "no".
But the argument for criminal liability is different because it does require subjective intent on the part of the perpetrator. A court could find that President Trump believed that the documents would be declassified simply because he took them, and so find that he had no criminal intent.
A D.C jury will not give him the benefit of the doubt.