You will be unsurprised to learn this about Farhad Manjoo's editor:
I asked [him] if he’s willing to consider making the single-page view the default on Slate. He said: “We should certainly consider making single-page views our default for articles below, say, 1200 words. That’s probably a good idea. I suspect it makes sense to keep paginating long articles.”
Eh, whaddya expect from a guy named Plotz.