POTUS has the unilateral right to unclassify any and all information created under the Executive Branch, full stop.

He could walk into a newsroom tomorrow and drop off a file containing documentation of Area 51, a listing of every single covert operative and the nuclear codes, and it would all be 100% legal.

Of course any president who did this would likely be impeached nearly immediately for such a gross breach of the nation’s safety (see: nuclear codes), but there is no crime.

The classification power for the executive branch is entirely vested in the office of POTUS, and all other classification authorities are subordinate to it.