I feel context is vitally important here, usernames are an important identify in who said what at what time, and can be used "on the record".
Twitter is not and has never been an officially sanctioned government press release channel. The consequences to government and policy would be trivial to none. Since most of the tweets from Trump are void of fact/research or are bizarre conspiracy theories I suspect a troll taking over the account would go largely unnoticed.
http://www.abajournal.com/news/article/government_says_trump...
In a more practical context I was in CENTCOM supporting troops in Syria when news of complete withdraw stunned the world via Twitter. The military did not act on a tweet. No formal change in policy in the government occurs via tweet.
In a more clear example remember back when Trump ordered, via tweet, the military will ban all transgender persons from military service? The military ignored that tweet without repercussion.
Does the military act on press releases? I would think both have importance based on how everyone else reacts to them, not because they serve as internal communication channels for the government.
Its like comparing the correspondence of a legal action from the government to a campaign rally. Just because many people in the public cannot tell the difference does not make them equivalent.
People who don't act on Trump's tweets can do so, but when he follows up by firing people who don't, that's something of a signal that they're meant to be taken as orders.
Point is, when you have 1) a tweet about an issue, 2) an official who says "that's not an order, we are going to wait until there's an order" followed by 3) that official being fired, it sure looks like the tweet was meant to be taken as an order.
https://mobile.twitter.com/steve_vladeck/status/119878175784...
Here is an article where Esper explains why he fired Spenser to Pentagon officials. https://townhall.com/tipsheet/cortneyobrien/2019/11/25/esper...