Yeah, that's the one. It's a great magazine, too bad about the editor's fetish for that letter since it's not part of the English orthography. I think it's also used in The New Yorker.
sigh Why can't English speakers just be happy with their language without trying to turn it into Latin or German or something else? It's particularly funny with the word "cooperate", since those editors are insisting on using a Germanic orthograph on a Latin-rooted word. Blegh, whatever. I don't usually care too much about language usage, but for some reason this one kind of gets under my skin.
The diaeresis in this context is not an Umlaut. It has similar roots as on ï which comes from French, I think. While äöü in German are letters with distinct sounds, the diaeresis on vowels in English usage signifies a short pause before that vowel.