I have not seen or listened to Trump’s interview and the only information I have about this is from what the media is reporting. However, I don’t think there is a formal process of declassifying documents, nor does it require a declassification stamp. This is something that was never really clarified in the past. The President may at his own discretion declassify documents, but there is no actual process that he is required to follow. Can he simply tell his staff that it’s declassified? Can he simply tell himself that it is declassified? Does he need to make a note or mark it as declassified?
Another thing that is interesting is, I don’t believe that Trump actually packed his own boxes and schlepped all those boxes out of the WH, he is a man of means and probably had other staff do it. There are lawyers and protocol people on staff that participate in everything that happens in the WH. This does not make him innocent, but it may complicate the story. He may not even have had any knowledge about the specifics of what was removed from the WH.