If he took them with him, wasn't he president until he left?
January 20. Did he stick around in the White house after that date? I don't think so.
Ergo, when he took them, he was still president.
FTR, I NEVER said everyone around President is incompetent.
Of course, he had to declassify them when he was still the president. But, others will likely claim otherwise. I believe President Trump and Kash Patel.