The President of the United States actively chooses to use @realDonaldTrump as in the same manner as a government official would in any other public forum, as such he must comply with the rules governing them as enshrined in the Constitution, features of that forum notwithstanding. The moment he began using it in a official capacity, was the moment it stopped being personal. Imagine if he was in a physical town hall, the placard in front of him doesn't change the fact that he's a government official.