I don’t think it’s that easy.
You cannot just unilaterally declare something declassified and then walk off with it in the next minute.
There are usually many checks & balances (remember them?) to keep people from having total power. They have to. It would be too dangerous. I cannot believe the President can just do that.
Believe what you want...seems to be How it is. Obama took thousands of documents. Doubt he was checked.
You and I can't. The president can.
It is that easy for the President. He is the ultimate Originating Authority who technically grants and rescinds all gov’t clearances but that is delegated to lower authorities for practical purposes. And he can classify and declassify anything.