The New Yorker is kind of well known for being virtually the only significant English publication which uses a diaeresis (the ¨) where almost everyone else uses a hyphen or just two repeated letters [0].
The diaeresis and umlaut are literally exactly the same Unicode character and the symbol or diacritic itself can be called either. The difference is what it does to the vowel — it’s a diaeresis if it indicates a repeated vowel, and an umlaut if it modifies the vowel sound.
Many of the N.Y.C. media outlets have style guides that may seem odd to other native speakers of American English. The one I find amusing is their insistence on using periods in acronyms and initialisms where no one else does.
Of course, N.Y.C. is probably just as correct as NYC, but they do it even when it is manifestly wrong.
One example is I.B.M. The major N.Y.C. media all spell it with periods, even though IBM itself does not use them.
Another is the SOS Morse Code distress signal, which the New Yorker spells S.O.S. with periods, seemingly in the belief that it is an initialism.
SOS is not an abbreviation or acronym or initialism at all. It is not even three separate letters when sent in Morse Code. It is a "prosign", which means that it is a single long Morse character, not sent as three separate letters with pauses between them.
It would be properly typeset with an overscore above the three letters, and no periods, as shown in the Wikipedia article. The overscore indicates that it is a single long character, and the letters are just a way to remember it.
I think they're just abusing their poetic license at this point, with the amount of times they choose to use archaic spellings and meanings of words, and I wish they would actually stop.
