Classification is one thing - the President can classify or declassify anything. But war plans are unquestionably national defense information, and Trump is charged with possessing and refusing to return national defense information, not with mishandling classified documents. Now it does not appear the Milley document is even the basis for one of the charges, but if it were, whether or not it was classified would be immaterial to the charge.
Milley accused Trump of wanting to go to war with Iran. Just because he created a military document to back up his lie doesn’t make it official national defense information.
Besides, the military still have national defense information on invading Narnia or Tatooine. LOL