“There will be no modifications to the current policy until the President's direction has been received by the Secretary of Defense and the Secretary has issued implementation guidance.” — Marine Gen. Joe Dunford
So the interesting thing here is what this means for civilian command. Tweeting is an unorthodox but theoretically valid means of communicating executive intent. The Joint Chiefs could have just as easily drafted a brief outlining a policy to implement the tweet, then the President could work with them to create that policy. They didn't do that, in fact they went on to draw a line: we can’t draft policy from tweets.
I think that, if Trump’s tweets routinely expressed workable policy recommendations, they would. And I think that might be a lawful & correct course of action. I would have opposed it on the merits, but that means we'd be debating the merits, not the mechanism.
Instead the Joint Chiefs issued a statement that they are waiting for “official” guidance from the White House. I don’t know whether JCs disagree with the policy direction. But it’s pretty clear they don’t think they can treat tweets as a clear communication of that direction.
I’m reminded of the numerous “sundowning” videos of Trump wandering around in public settings. I don’t believe he is actually experiencing dementia — or rather that’s not the point. The point is: secret service & other handlers LET the president walk wherever he wants: away from the limo, out of the room without shaking Bibi's hand, out from behind the bullet barrier, WHEREVER. It always takes his entourage a few seconds to realize he’s headed out of bounds and they step in to steer him back. (Again the REASON he’s doing this is not important to my point.) The assumption is that the President knows his own intent and is consciously acting on it. That's been the operating assumption for 240 years!
By design, we don't have a lot of mechanisms in government to constrain people to norms. Norms like “the President knows what he wants, and can communicate it to the government.” Someone who violates those norms isn't just pissing off fuddy-duddies, they are creating ambiguities the system can't resolve. Which leads (for example) the Joint Chiefs to issue a statement that’s probably at odds with the intent of the Commander In Chief.