It's very likely Dropbox has changed some things since then, having grown to the point where many problems have likely gone from "just buy more hardware" to "let's make this more efficient" in solution-space.
Because I would like to know about scalability issues?
Perhaps in these two years their userbase and the number files saved every 15 minutes might have grown by, say 5x and 50x, revealing problems that were not visible in 2011, leading them to modify the system from what is mentioned in the article?