Threatening other people?
It is however illegal for me to do it.
And that's likely where the TOS rule comes from.
We're struggling with a spirit of the law, letter of the law issue here.
The alternatives are either granting all parties equal right to any chosen violence they choose to enact, granting the right to some particular entity, or attempting ... and by what means specifically without a state and sanction of legitimate violence ... to bar all use of violence.
Note too: legitimacy does not mean unlimited or capricious. It does, however, mean that the sanction of violence originates from, and is enacted either directly by, or through licence (e.g., self-defence) from, the state.
Misuse of this term is rampant, particularly among so-called Libertarians.
I think the "legitimate" is implied - a State is a thing that creates Legitimacy and enforces it through Violence. To supplement this it monopolizes Violence by inflicting Violence on those who commit it without Legitimacy.
The fact was an insight to me when I happened to stumble across it (researching other aspects of Weber, as it happens).
Twitter is not the court or the police.
It's a private company.
The President can threaten violence (and should not be hindered by private companies to do so) because he was elected to be able to do so. Whether you agree with his ability to kill in defense (etc.) or not.
---
Any citizen of the United States, wherever he may be, who, without authority of the United States, directly or indirectly commences or carries on any correspondence or intercourse with any foreign government or any officer or agent thereof, with intent to influence the measures or conduct of any foreign government or of any officer or agent thereof, in relation to any disputes or controversies with the United States, or to defeat the measures of the United States, shall be fined under this title or imprisoned not more than three years, or both.
---
The TOS is clearly based on law, it's legal for him to do it, so in essence the TOS regarding to his legal rights, shouldn't (in spirit) apply to him.
Now if he was doing something that he doesn't have the right to do, i.e. inciting violence against a private person, he'll break the TOS.
If all you want to say is that what he has done is strictly against the text of the TOS then sure I'll agree. But I don't think it matters.
If he tells a foreign official anything at any point in time whether it be email, phone call, twitter etc. it's official. He can't claim that he was taking a break. He can delete a tweet, but that doesn't mean that it didn't happen.