He wasn't a private citizen when he had them shipped to Mar A Lago. He was President.
However, he claimed that he declassified them AFTER he left office.
Since there is no proof that he did, the law will continue to recognize the documents as classified.
But, for an Espionage Charge, it won't matter, one way or the other. DNI damage assessments will carry far more weight, and those assessments also do not depend on classification.