You're only half right. Yes, the sitting president can declassify anything they want to, it's their perogative as president. HOWEVER, those classified documents are also held at other places as classified documents. When the president says that something is declassified, there is a memo that goes out to all the other places where the information is held stating that it has been declassified. Without a memo, all we have is Trump saying that he did something, but in reality, he did nothing, and the documents are still classified. And, he's just now saying this, probably as a way to hopefully keep himself out of jail. He's not the current sitting president, and those documents are still classified. Otherwise, there would be a memo stating that the material in question had been declassified, along with the date such action was taken.