The New Yorker's house style is to use a dieresis to indicate when adjacent vowels do not form a diphthong. There's a syllable-break in between the Os of "coöperate", as opposed to between the Os of "chicken coop", so it gets a special marker.
Just to complete the trifecta, the URL date is presumably that of the print issue in which this article will appear. This "cover date" is usually a week or so ahead of the actual printing date: https://en.wikipedia.org/wiki/Cover_date
See https://en.wikipedia.org/wiki/Diaeresis_%28diacritic%29#Engl...
"proffer" is also a perfectly legitimate word: https://www.merriam-webster.com/dictionary/proffer