It may not be unusual, but the Tweet can still be legally used against him. By this point, he has adopted the words of an authorized agent by not deleting the Tweet. And those words essentially admitted to obstructing justice. If one of Trump's lawyers really did Tweet this, that lawyer is also risking disbarment. It's different than you Tweeting for your CEO--unless I'm mistaken, you are not a legal representative and agent for your CEO. This wasn't Trump's PR person making a statement, it was allegedly his lawyer. Regardless of who is Tweeting with Trump's account, these words can be used against Trump. But it is unlikely anything will come of this because our country is hopelessly broken at this point.