Sorry -- that's not mentioned in the post, but that's precisely how it works. Nginx handles HTTPS, and Rails only ever sees HTTP, with the "X-FORWARDED-PROTO" header.
Sorry, I didn't get that from the article. I use nginx to do all my redirects to https if someone changes an https link to http. But, that means keeping the nginx config up-to-date.
From the article: "Nginx handles the SSL itself, in a straightforward fashion, as long as you’ve compiled it with the SSL module turned on."