Steven Aftergood, director of the Federation of American Scientists Project on Government Secrecy, said that such authority gives the president the authority to "classify and declassify at will." The president is not "obliged to follow any procedures other than those that he himself has prescribed," Aftergood said. "And he can change those."
"There’s no question that the president has broad authority to declassify almost anything at any time without any process," said Stephen I. Vladeck, professor at the University of Texas School of Law.
https://www.politifact.com/factchec...resident-have-ability-declassify-anything-an/
Your same source weighed in on the current investigation:
https://www.politifact.com/article/2022/aug/11/could-trump-argue-declassified-documents/
Could Trump argue that he declassified certain documents in private, while president? That is not how the system is designed to work.
"Merely proclaiming a document or group of documents declassified and doing nothing more would not suffice," Bradley Moss, a Washington, D.C.-based lawyer who works on national security cases, told PolitiFact.
Follow-through is required.
"He had to identify the specific documents he was declassifying, he needed to memorialize the order in writing for bureaucratic and historical purposes, and he needed to have staff physically modify the classification markings on the documents themselves," Moss said. "Until that was done, the documents, per the security classification procedures, still have to be handled, transmitted and stored as if they were classified."
Tom Blanton, director of the National Security Archive at George Washington University, agreed.
"If the documents are still marked classified 18 months after their removal from the White House," Blanton told PolitiFact, "then Trump was too busy to order them declassified at the time."