The Espionage Act violations are for people who shouldn't have sensitive material. The problem here is that a President of the United States knowingly put the materials there in Mar-a-Lago for a former president to keep. This makes it legitimate for Trump to have the papers there, by intent of a US President.
The problem is that there is an audio recording where Trump admitted that he didn't declassify some of the material at Mar-a-Lago while he was president, therefore it was not legitimate for him to have those materials.
It is possible that it still might be confidential or secret, despite whether Trump gave himself clearance and determined that they should be in his possession post-Presidency. The act of moving the papers to Mar-a-Lago is merely an implicit act of approving himself to have those papers, declassifying it to an extent for himself perhaps, but not necessarily broad declassification.
If a president motions a guest in the White House into a classified military briefing, the president is essentially declassifying the material to an uncleared person, but it doesn't make the whole military operation unclassified. It makes that person cleared to view that material on a need-to-know basis. If the classified material gets out to someone else, that person can still be arrested for having classified material.
However, the audio recording is pretty vague about what it is talking about. He could be talking about something that is unclassified, but marked "confidential", which is "secret", but different than an official document marked secret -
https://www.youtube.com/watch?v=9IetYamtgU4&ab_channel=CNN
If it is secret, then President Trump has clearly authorized Former President Trump to have it in his possession by moving the papers to Mar-a-Lago. The fact that he was once president and did this, and the lack of policies in presidential classification and declassification procedures, means that this is an exercise in futility.