He means Hacker News' next page link. The URL you get by clicking it is unique to you and it preserves order of stories when you navigate from one page to next, but it has short lifetime.
Yes, that's right, but if you look at the page source you'll see fnid used elsewhere too, e.g. "add comment" IIRC. It's annoying to have typed a comment, hit add, only to find the continuation has been flushed from the cache at the server.