The argument that @realDonaldTrump was viewpoint discrimination is because that twitter account was being used as a defacto mouthpiece of the US government. As

The argument that @realDonaldTrump was viewpoint discrimination is because that twitter account was being used as a defacto mouthpiece of the US government. As such, people engaging with it are entitled to 1A protection. That doesn't mean that Donald Trump is entitled, either as an individual, or as POTUS, to have a Twitter account.