It is not clear that the President can unilaterally declassify everything, especially nuclear weapons materials. Not sure how it would stand up in court, but there is a law saying he would need the OK from DOE.
But either way, if he had TS stuff in his possession after 1/20/21, then he was in violation.
It is a mystery about why he would, or what stuff he took. Obviously.
It will all be revealed in due time.