[Manheim]

I wouldn't say that. From the article; "The text was forwarded by the White House National Security Council to ambassadors in Washington, and was clearly intended to be widely shared. Here it is":

Dear Jonas:

Considering your Country decided not to give me the Nobel Peace Prize for having stopped 8 Wars PLUS, I no longer feel an obligation to think purely of Peace, although it will always be predominant, but can now think about what is good and proper for the United States of America. Denmark cannot protect that land from Russia or China, and why do they have a “right of ownership” anyway? There are no written documents, it’s only a boat that landed there hundreds of years ago, but we had boats landing there, also. I have done more for NATO than any person since its founding, and now, NATO should do something for the United States. The World is not secure unless we have Complete and Total Control of Greenland. Thank you! President DJT

[eudamoniac]

The article is an opinion piece that cites a tweet. Primary sources seem to only call it "a text message"

[Manheim]

For your information: The Norwegian Government refers to it as a letter from the President of the United States to the Prime Minister of Norway. So no, this is not a random tweet.

If I remember correctly, the message was originally sent as a text message, but when a head of state sends a written message to another head of state, and that message is then circulated as an official FYI to the sender’s ambassadors, it is widely regarded as official communication.

The article itself is an opinion piece, yes, but the letter has been extensively reported across Europe and internationally. I could just as well have referenced one of those reports.