This is the only explanation we can give for this sentence, which is positively screaming for a comma (and maybe the sort of surgery that separates conjoined twins):
The matter is not simply that his writers were on the picket line on a primary eve that saw both a formerly fat former governor of Arkansas introduce a sandwich called the Huckaburger and Mike Gravel, the presently zany former governor of Alaska, advise an audience at Phillips Exeter Academy to smoke marijuana, as if boarding-school students needed such encouragement.
The sandwich is called "the Huckaburger." It's not "the Huckaburger and Mike Gravel," even though that's how anyone not from a town called Hope would read it. When a sentence sprouts a complete second half, you need to insert a comma before the conjunction.
Sometimes, when you have two complete sentences joined by a conjunction, you can drop the conjunction and insert a semicolon, but not in this particular example, or on Slate in general because its former editor, Michael Kinsley, hates semicolons. That's awfully rich for a guy who was still wearing handsome little gold-rimmed spectacles in the 1990s.