He wrote a memo dated Jan 19, 2109 declassifying what he had at MAL. I believe that means he was President on that date.
If you're referring to the Crossfire Hurricane memo, that actually makes his legal position worse than if it didn't exist at all for a few reasons:
1) It's a formal, written and signed memo declassifying specific documents related to Crossfire Hurricane, and to no other documents:
https://trumpwhitehouse.archives.gov/presidential-actions/memorandum-declassification-certain-materials-related-fbis-crossfire-hurricane-investigation/2. The existence of the memo proves that when Trump wanted to declassify documents, he knew exactly how to do it. He didn't just say after he left office that "by the way, I declassified those Crossfire Hurricane documents in my head". He had it written down and signed while he was still President so nobody could dispute that he had taken an official Presidential action. That's just common sense, right? It also ensures that the entities within the government responsible for tracking and safeguarding that classified information know that it is no longer classified. They won't know it if he doesn't tell them.
3. Given that he signed this particular declassification memo on January 19, left office the very next day and took all those other documents with him, it begs the question of why - if he truly did intend to declassify all those other documents -- he didn't just do the exact same thing he did with the Crossfire Hurricane documents? He's got a huge staff, lawyers, etc.. All he had to do was say "do up another memo applying to all these other documents I'm taking so I can sign that one too." But he didn't. So the question is...why?
The most logical conclusion to be drawn from that is that he had
not decided to declassify all those other document.
3. If Trump truly believed that he could declassify documents simply by "thought", he wouldn't have needed that Crossfire Hurricane memo in the first place.
I think one thing that is very important to understand is that when documents are declassified, that applies to everyone, and to every copy of the document. That means anyone in the government could make the information publicly without being accused of disclosing classified material. So if Trump actually had said "I am officially declassifying all those documents", he's basically letting bad guys get that information. And presumably, there is some information in there that he did
not want bad guys to get access to. So, he didn't declassify those documents. He just took them with him while they were still classified, and therefore would continue to be treated as such by the government.