Notice that the source is Twitter. Is Twitter a legally accepted source of legal presidential speech? In other words, if Trump gets on Twitter, and orders the military to do military exercises, does the military have to obey if there is no standard form of orders sent by the President to them through standard military channels?
Anybody can get on Twitter and say the same things that Trump did. So what? There is no harm done against anybody. Name the man or woman in Trump's threat. Iran is a corporation of sorts. It isn't any man or woman. And Trump did absolutely nothing legal or illegal that threatens any man or woman through his Tweet.
If somebody wants to be scared of what Trump said, that's his stupidity, not Trump's. After all, it is Trump the man who Tweets... not Trump the President.

Trump tweets are considered 'official statements'. Same as if he addressed the Nation on TV or talked to a reporter or released a written statement.