You're right, a president can declassify anything they want to (or in your words "at will"). BUT, they still need to let everyone else holding the information know that it has been declassified. It can't be declassified in Trump's possession, yet classified in other places, that's not how classified material works. And, if nobody else knows that the information was declassified (via the memo sent out by his staff) then they are required to treat it as classified. Classified information is not Schrodenger's cat, it can't exist in 2 states at once (alive/dead, classified/declassified), it has to be one or the other.