I doubt that a tweet would suffice. The tweet used the past tense as if describing an act that had already taken place. Is the President allowed to lie in a tweet? Probably.
Some of us remember the so-called Saturday Night Massacre, in which Nixon was forced to immediately accept the resignations of two officials before he found one willing to fire a Special Prosecutor.
If Trump wanted documents immediately declassified and released he has the power to accomplish it within the hour. One must conclude that he doesn't actually want that.
There are only two requirements for an order. Was it communicated and was it lawful [and orders carry the rebuttable presumption of lawfulness].