You've been misled by the publisher. Seveneves has about 600 pages. After that, the pages are filled with some kind of fan fiction sort-a related, but pretty dumb, novella (confusingly named "Part Three").

Now, seriously. While the first two parts are great, the third is awful. You can actually skip the part 3, it is completely superfluous (and again, dumb), the story works without it better.