Exactly. Imagine how the intelligence community would have to treat classified information if it could be declassified purely by a President waking up and thinking “it’s declassified.” Nobody would know what is still classified and what isn’t. Classified information might get leaked out because of a rumor that the President declassified something. Declassified information might still be treated as classified by some because nobody knows what is and what isn’t classified.
There’s a process for a reason. Trump suggesting he can just “think” it declassified is totally ridiculous.