I'll assume the veracity of that because I don't have a ton of interest in Trump personally and get weary when every topic of discussion is hauled back to him, but if he maintained that the material was still classified at the moment Biden was inaugurated (and presumably remained so thereafter) then it would probably become a twofold issue of whether a) the material was disclosed to unauthorized persons and b) whether the material was stored in a way such that it couldn't be accessed by unauthorized individuals. If a) was the case I'd think even a former POTUS would potentially be in legal jeopardy. If the issue is only with b) in the sense the material was not stored in an approved manner then it would generally be more of an administrative thing. At this point Doug and tj probably have more expertise/relevant experience than I do.
(I realize there's more to that case w/Trump than just his possession of a document/documents, namely his alleged noncooperation in the matter, and I suppose also that his clearance was revoked by Biden at some point in the timeline. Maybe it will get litigated one day.)
But I still stand by what I initially said that only a POTUS can spontaneously declassify something, and I believe that because I saw it happen and got an explanation from officials whose position in the gov't was to enforce laws and regulations pertaining to formally classified information per the EO (forget the number) that lays out the basis for the classification system. I don't see how whether or not a POTUS did or didn't decide something was unclassified before he left office has any bearing on Bolton's situation. The topic before it got hijacked followed from someone sharing an X posting that claimed Bolton was being looked at for a pattern of "stealing" classified information and leaking it. Probably mostly BS, but maybe not. We'll see what happens. Either he did or he didn't, and since he was never POTUS, he never had the authority to do what I watched a president prior to Trump do, making it considerably more black-and-white in the event some evidence against him exists.