If he were president right now, you'd have a point. He could keep classified records in the Oval Office or take them and read them while sunbathing on the beach because he'd be presidentin'.
The idea that he can retroactively declare them classified is foolish - no rational judge or legal expert would agree with you. The idea that he can claim that he declared them classified without any hard proof is also foolish. Those documents aren't Trump's personal property - they're US government property. They are not his to declare open to the public, and not his to keep. There's a process for deciding which documents can be open to public view and he didn't follow it, so they're classified.