Thing is that its not up to the FBI or the Pentagon or others as the ultimate authority. The President is unique in this as the CiC. I really dont understand how you can apply a breach of the secrecy bits to any President. Forget the fine details for a moment, Scotus made it pretty clear during Clinton's time (just re-iterating well worn precedent actually) that a President has almost unlimited authority over classified docs. They can really do near anything they want.
That does NOT include VPs though like Pence or Biden was, nor anyone else in government, for whom all the rules apply. These rules really DONT apply to a President - whether Clinton, Bush, Obama or Trump. They are special. Obama has TONS of docs he took with him that were not mentioned.
And all these docs were packed up by GSA and catalogued and moved to Miami - National Archives KNEW about them and where they were. The Biden docs they had NO IDEA existed or the location.