I see nothing in the judges description about classified anything. I did see that the historian is the one who did the recording so technically it would have been his property at the time.
[D]ocumentary materials, or any reasonably segregable portion thereof, created or received by the President, his immediate staff, or a unit or individual of the Executive Office of the President whose function is to advise and assist the President, in the course of conducting activities which relate to or have an effect upon the carrying out of the constitutional, statutory, or other official or ceremonial duties of the President.
This is what the statute says. The historian does not fall under any of these categories of people listed.
In making a decision on this matter I have to consider the nature of the audio tapes, if they were created with the intent of their use as government materials, and whether or not they were circulated within the Administration or relied on as policy documents. On the facts made available to me, I do not believe the materials in question fall within the ambit of the PRA.
This is the most relevant part of the decision. If you want to apply this standard to the documents Trump had. I think thats just fine.